linux kernel RCU 以及读写锁
信号量有一个很明显的缺点，没有区分临界区的读写属性，读写锁允许多个线程进程并发的访问临界区，但是写访问只限于一个线程，在多处理器系统中允许多个读者访问共享资源，但是写者有排他性，读写锁的特性如下：允许多个读者同时访问临界区，但是同一时间不能进入；同一时刻只允许一个写者进入临界区；读者和写者不能同时进入临界区。读写锁也有关闭中断和下半部的版本。
RCU:read-copy-update 。。。。。。。。。。。。。。。。。。。。
问题：rcu相比读写锁，解决了什么问题？ rcu的基本原理？
1、由于内核中spinlock mutex 等都使用了原子操作指令，即原子的访问内存，但是当多cpu 竞争访问临界区时会让cpu的cache命中率下降，性能下降。同时读写锁有个缺陷，读者和写者不能同时存在。
rcu实现的目标就是要解决这个问题，为了使线程同步开销小。不需要原子操作以及内存屏障而访问数据，把同步的问题交给写者线程，写者线程等待所有的读者线程完成后才会吧旧数据销毁。当有多个写者线程存在时，需要额外的保护机制。
原理
RCU原理：简单理解为 记录了所有指向共享数据的指针使用者，当要修改共享数据时，先创建一个副本，在副本中修改。所有读者离开临界区后，指针指向新的修改副本后的地方，并且删除旧数据。
官方描述：RCU实际上是一种改进的rwlock，读者几乎没有什么同步开销，它不需要锁，不使用原子指令，因此不会导致锁竞争，内存延迟以及流水线停滞。不需要锁也使得使用更容易，因为死锁问题就不需要考虑了。
- 写者的同步开销比较大，它需要延迟数据结构的释放，复制被修改的数据结构，它也必须使用某种锁机制同步并行的其它写者的修改操作。
- 读者必须提供一个信号给写者以便写者能够确定数据可以被安全地释放或修改的时机。
- 有一个专门的垃圾收集器来探测读者的信号，一旦所有的读者都已经发送信号告知它们都不在使用被RCU保护的数据结构，垃圾收集器就调用回调函数完成最后的数据释放或修改操作。
目前在内核中链表使用RCU较多。
在经典RCU中，RCU读侧临界部分由rcu_read_lock() 和rcu_read_unlock()界定，它们可以嵌套。
对应的同步更新原语为synchronize_rcu()，还有同义的synchronize_net()，等待当前正执行的RCU读侧闻临界部分运行完成。等待的时间称为“宽限期”。
异步更新侧原语call_rcu()在宽限期之后触发指定的函数，如：call_rcu(p,f)调用触发回调函数f(p)。有些情况，如：当卸载使用call_rcu()的模块，必须等待所有RCU回调函数完成，原语rcu_barrier()起该作用。
在“RCU BH”列中，rcu_read_lock_bh() 和rcu_read_unlock_bh()界定读侧临界部分，call_rcu_bh()在宽限期后触发指定的函数。注意：RCU BH没有同步接口synchronize_rcu_bh()，如果需要，用户很容易添加同步接口函数。
直接操作指针的原语rcu_assign_pointer()和rcu_dereference()用于创建RCU保护的非链表数据结构，如：数组和树
NOTE：读者在访问被RCU保护的共享数据期间不能被阻塞，这是RCU机制得以实现的一个基本前提，也就说当读者在引用被RCU保护的共享数据期间，读者所在的CPU不能发生上下文切换，spinlock和rwlock都需要这样的前提。 写者在访问被RCU保护的共享数据时不需要和读者竞争任何锁，只有在有多于一个写者的情况下需要获得某种锁以与其他写者同步。写者修改数据前首先拷贝一个被修改元素的副本，然后在副本上进行修改，修改完毕后它向垃圾回收器注册一个回调函数以便在适当的时机执行真正的修改操作。等待适当时机的这一时期称为宽限期（grace period），而CPU发生了上下文切换称为经历一个quiescent state，grace period就是所有CPU都经历一次quiescent state所需要的等待的时间。垃圾收集器就是在grace period之后调用写者注册的回调函数来完成真正的数据修改或数据释放操作的。
/* Please note that the "What is RCU?" LWN series is an excellent place to start learning about RCU: 1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/ 2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/ 3. RCU part 3: the RCU API http://lwn.net/Articles/264090/ 4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/ What is RCU? RCU is a synchronization mechanism that was added to the Linux kernel during the 2.5 development effort that is optimized for read-mostly situations. Although RCU is actually quite simple once you understand it, getting there can sometimes be a challenge. Part of the problem is that most of the past descriptions of RCU have been written with the mistaken assumption that there is "one true way" to describe RCU. Instead, the experience has been that different people must take different paths to arrive at an understanding of RCU. This document provides several different paths, as follows: 1. RCU OVERVIEW 2. WHAT IS RCU'S CORE API? 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? 6. ANALOGY WITH READER-WRITER LOCKING 7. FULL LIST OF RCU APIs 8. ANSWERS TO QUICK QUIZZES People who prefer starting with a conceptual overview should focus on Section 1, though most readers will profit by reading this section at some point. People who prefer to start with an API that they can then experiment with should focus on Section 2. People who prefer to start with example uses should focus on Sections 3 and 4. People who need to understand the RCU implementation should focus on Section 5, then dive into the kernel source code. People who reason best by analogy should focus on Section 6. Section 7 serves as an index to the docbook API documentation, and Section 8 is the traditional answer key. So, start with the section that makes the most sense to you and your preferred method of learning. If you need to know everything about everything, feel free to read the whole thing -- but if you are really that type of person, you have perused the source code and will therefore never need this document anyway. ;-) 1. RCU OVERVIEW The basic idea behind RCU is to split updates into "removal" and "reclamation" phases. The removal phase removes references to data items within a data structure (possibly by replacing them with references to new versions of these data items), and can run concurrently with readers. The reason that it is safe to run the removal phase concurrently with readers is the semantics of modern CPUs guarantee that readers will see either the old or the new version of the data structure rather than a partially updated reference. The reclamation phase does the work of reclaiming (e.g., freeing) the data items removed from the data structure during the removal phase. Because reclaiming data items can disrupt any readers concurrently referencing those data items, the reclamation phase must not start until readers no longer hold references to those data items. Splitting the update into removal and reclamation phases permits the updater to perform the removal phase immediately, and to defer the reclamation phase until all readers active during the removal phase have completed, either by blocking until they finish or by registering a callback that is invoked after they finish. Only readers that are active during the removal phase need be considered, because any reader starting after the removal phase will be unable to gain a reference to the removed data items, and therefore cannot be disrupted by the reclamation phase. So the typical RCU update sequence goes something like the following: a. Remove pointers to a data structure, so that subsequent readers cannot gain a reference to it. b. Wait for all previous readers to complete their RCU read-side critical sections. c. At this point, there cannot be any readers who hold references to the data structure, so it now may safely be reclaimed (e.g., kfree()d). Step (b) above is the key idea underlying RCU's deferred destruction. The ability to wait until all readers are done allows RCU readers to use much lighter-weight synchronization, in some cases, absolutely no synchronization at all. In contrast, in more conventional lock-based schemes, readers must use heavy-weight synchronization in order to prevent an updater from deleting the data structure out from under them. This is because lock-based updaters typically update data items in place, and must therefore exclude readers. In contrast, RCU-based updaters typically take advantage of the fact that writes to single aligned pointers are atomic on modern CPUs, allowing atomic insertion, removal, and replacement of data items in a linked structure without disrupting readers. Concurrent RCU readers can then continue accessing the old versions, and can dispense with the atomic operations, memory barriers, and communications cache misses that are so expensive on present-day SMP computer systems, even in absence of lock contention. In the three-step procedure shown above, the updater is performing both the removal and the reclamation step, but it is often helpful for an entirely different thread to do the reclamation, as is in fact the case in the Linux kernel's directory-entry cache (dcache). Even if the same thread performs both the update step (step (a) above) and the reclamation step (step (c) above), it is often helpful to think of them separately. For example, RCU readers and updaters need not communicate at all, but RCU provides implicit low-overhead communication between readers and reclaimers, namely, in step (b) above. So how the heck can a reclaimer tell when a reader is done, given that readers are not doing any sort of synchronization operations??? Read on to learn about how RCU's API makes this easy. 2. WHAT IS RCU'S CORE API? The core RCU API is quite small: a. rcu_read_lock() b. rcu_read_unlock() c. synchronize_rcu() / call_rcu() d. rcu_assign_pointer() e. rcu_dereference() There are many other members of the RCU API, but the rest can be expressed in terms of these five, though most implementations instead express synchronize_rcu() in terms of the call_rcu() callback API. The five core RCU APIs are described below, the other 18 will be enumerated later. See the kernel docbook documentation for more info, or look directly at the function header comments. rcu_read_lock() void rcu_read_lock(void); Used by a reader to inform the reclaimer that the reader is entering an RCU read-side critical section. It is illegal to block while in an RCU read-side critical section, though kernels built with CONFIG_PREEMPT_RCU can preempt RCU read-side critical sections. Any RCU-protected data structure accessed during an RCU read-side critical section is guaranteed to remain unreclaimed for the full duration of that critical section. Reference counts may be used in conjunction with RCU to maintain longer-term references to data structures. rcu_read_unlock() void rcu_read_unlock(void); Used by a reader to inform the reclaimer that the reader is exiting an RCU read-side critical section. Note that RCU read-side critical sections may be nested and/or overlapping. synchronize_rcu() void synchronize_rcu(void); Marks the end of updater code and the beginning of reclaimer code. It does this by blocking until all pre-existing RCU read-side critical sections on all CPUs have completed. Note that synchronize_rcu() will -not- necessarily wait for any subsequent RCU read-side critical sections to complete. For example, consider the following sequence of events: CPU 0 CPU 1 CPU 2 ----------------- ------------------------- --------------- 1. rcu_read_lock() 2. enters synchronize_rcu() 3. rcu_read_lock() 4. rcu_read_unlock() 5. exits synchronize_rcu() 6. rcu_read_unlock() To reiterate, synchronize_rcu() waits only for ongoing RCU read-side critical sections to complete, not necessarily for any that begin after synchronize_rcu() is invoked. Of course, synchronize_rcu() does not necessarily return -immediately- after the last pre-existing RCU read-side critical section completes. For one thing, there might well be scheduling delays. For another thing, many RCU implementations process requests in batches in order to improve efficiencies, which can further delay synchronize_rcu(). Since synchronize_rcu() is the API that must figure out when readers are done, its implementation is key to RCU. For RCU to be useful in all but the most read-intensive situations, synchronize_rcu()'s overhead must also be quite small. The call_rcu() API is a callback form of synchronize_rcu(), and is described in more detail in a later section. Instead of blocking, it registers a function and argument which are invoked after all ongoing RCU read-side critical sections have completed. This callback variant is particularly useful in situations where it is illegal to block or where update-side performance is critically important. However, the call_rcu() API should not be used lightly, as use of the synchronize_rcu() API generally results in simpler code. In addition, the synchronize_rcu() API has the nice property of automatically limiting update rate should grace periods be delayed. This property results in system resilience in face of denial-of-service attacks. Code using call_rcu() should limit update rate in order to gain this same sort of resilience. See checklist.txt for some approaches to limiting the update rate. rcu_assign_pointer() typeof(p) rcu_assign_pointer(p, typeof(p) v); Yes, rcu_assign_pointer() -is- implemented as a macro, though it would be cool to be able to declare a function in this manner. (Compiler experts will no doubt disagree.) The updater uses this function to assign a new value to an RCU-protected pointer, in order to safely communicate the change in value from the updater to the reader. This function returns the new value, and also executes any memory-barrier instructions required for a given CPU architecture. Perhaps just as important, it serves to document (1) which pointers are protected by RCU and (2) the point at which a given structure becomes accessible to other CPUs. That said, rcu_assign_pointer() is most frequently used indirectly, via the _rcu list-manipulation primitives such as list_add_rcu(). rcu_dereference() typeof(p) rcu_dereference(p); Like rcu_assign_pointer(), rcu_dereference() must be implemented as a macro. The reader uses rcu_dereference() to fetch an RCU-protected pointer, which returns a value that may then be safely dereferenced. Note that rcu_deference() does not actually dereference the pointer, instead, it protects the pointer for later dereferencing. It also executes any needed memory-barrier instructions for a given CPU architecture. Currently, only Alpha needs memory barriers within rcu_dereference() -- on other CPUs, it compiles to nothing, not even a compiler directive. Common coding practice uses rcu_dereference() to copy an RCU-protected pointer to a local variable, then dereferences this local variable, for example as follows: p = rcu_dereference(head.next); return p->data; However, in this case, one could just as easily combine these into one statement: return rcu_dereference(head.next)->data; If you are going to be fetching multiple fields from the RCU-protected structure, using the local variable is of course preferred. Repeated rcu_dereference() calls look ugly, do not guarantee that the same pointer will be returned if an update happened while in the critical section, and incur unnecessary overhead on Alpha CPUs. Note that the value returned by rcu_dereference() is valid only within the enclosing RCU read-side critical section. For example, the following is -not- legal: rcu_read_lock(); p = rcu_dereference(head.next); rcu_read_unlock(); x = p->address; /* BUG!!! */ rcu_read_lock(); y = p->data; /* BUG!!! */ rcu_read_unlock(); Holding a reference from one RCU read-side critical section to another is just as illegal as holding a reference from one lock-based critical section to another! Similarly, using a reference outside of the critical section in which it was acquired is just as illegal as doing so with normal locking. As with rcu_assign_pointer(), an important function of rcu_dereference() is to document which pointers are protected by RCU, in particular, flagging a pointer that is subject to changing at any time, including immediately after the rcu_dereference(). And, again like rcu_assign_pointer(), rcu_dereference() is typically used indirectly, via the _rcu list-manipulation primitives, such as list_for_each_entry_rcu(). The following diagram shows how each API communicates among the reader, updater, and reclaimer. rcu_assign_pointer() +--------+ +---------------------->| reader |---------+ | +--------+ | | | | | | | Protect: | | | rcu_read_lock() | | | rcu_read_unlock() | rcu_dereference() | | +---------+ | | | updater |<---------------------+ | +---------+ V | +-----------+ +----------------------------------->| reclaimer | +-----------+ Defer: synchronize_rcu() & call_rcu() The RCU infrastructure observes the time sequence of rcu_read_lock(), rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in order to determine when (1) synchronize_rcu() invocations may return to their callers and (2) call_rcu() callbacks may be invoked. Efficient implementations of the RCU infrastructure make heavy use of batching in order to amortize their overhead over many uses of the corresponding APIs. There are no fewer than three RCU mechanisms in the Linux kernel; the diagram above shows the first one, which is by far the most commonly used. The rcu_dereference() and rcu_assign_pointer() primitives are used for all three mechanisms, but different defer and protect primitives are used as follows: Defer Protect a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock() call_rcu() rcu_dereference() b. synchronize_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh() call_rcu_bh() rcu_dereference_bh() c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched() call_rcu_sched() preempt_disable() / preempt_enable() local_irq_save() / local_irq_restore() hardirq enter / hardirq exit NMI enter / NMI exit rcu_dereference_sched() These three mechanisms are used as follows: a. RCU applied to normal data structures. b. RCU applied to networking data structures that may be subjected to remote denial-of-service attacks. c. RCU applied to scheduler and interrupt/NMI-handler tasks. Again, most uses will be of (a). The (b) and (c) cases are important for specialized uses, but are relatively uncommon. 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? This section shows a simple use of the core RCU API to protect a global pointer to a dynamically allocated structure. More-typical uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt. struct foo { int a; char b; long c; }; DEFINE_SPINLOCK(foo_mutex); struct foo __rcu *gbl_foo; /* * Create a new struct foo that is the same as the one currently * pointed to by gbl_foo, except that field "a" is replaced * with "new_a". Points gbl_foo to the new structure, and * frees up the old structure after a grace period. * * Uses rcu_assign_pointer() to ensure that concurrent readers * see the initialized version of the new structure. * * Uses synchronize_rcu() to ensure that any readers that might * have references to the old structure complete before freeing * the old structure. */ void foo_update_a(int new_a) { struct foo *new_fp; struct foo *old_fp; new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL); spin_lock(&foo_mutex); old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex)); *new_fp = *old_fp; new_fp->a = new_a; rcu_assign_pointer(gbl_foo, new_fp); spin_unlock(&foo_mutex); synchronize_rcu(); kfree(old_fp); } /* * Return the value of field "a" of the current gbl_foo * structure. Use rcu_read_lock() and rcu_read_unlock() * to ensure that the structure does not get deleted out * from under us, and use rcu_dereference() to ensure that * we see the initialized version of the structure (important * for DEC Alpha and for people reading the code). */ int foo_get_a(void) { int retval; rcu_read_lock(); retval = rcu_dereference(gbl_foo)->a; rcu_read_unlock(); return retval; } So, to sum up: o Use rcu_read_lock() and rcu_read_unlock() to guard RCU read-side critical sections. o Within an RCU read-side critical section, use rcu_dereference() to dereference RCU-protected pointers. o Use some solid scheme (such as locks or semaphores) to keep concurrent updates from interfering with each other. o Use rcu_assign_pointer() to update an RCU-protected pointer. This primitive protects concurrent readers from the updater, -not- concurrent updates from each other! You therefore still need to use locking (or something similar) to keep concurrent rcu_assign_pointer() primitives from interfering with each other. o Use synchronize_rcu() -after- removing a data element from an RCU-protected data structure, but -before- reclaiming/freeing the data element, in order to wait for the completion of all RCU read-side critical sections that might be referencing that data item. See checklist.txt for additional rules to follow when using RCU. And again, more-typical uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt. 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? In the example above, foo_update_a() blocks until a grace period elapses. This is quite simple, but in some cases one cannot afford to wait so long -- there might be other high-priority work to be done. In such cases, one uses call_rcu() rather than synchronize_rcu(). The call_rcu() API is as follows: void call_rcu(struct rcu_head * head, void (*func)(struct rcu_head *head)); This function invokes func(head) after a grace period has elapsed. This invocation might happen from either softirq or process context, so the function is not permitted to block. The foo struct needs to have an rcu_head structure added, perhaps as follows: struct foo { int a; char b; long c; struct rcu_head rcu; }; The foo_update_a() function might then be written as follows: /* * Create a new struct foo that is the same as the one currently * pointed to by gbl_foo, except that field "a" is replaced * with "new_a". Points gbl_foo to the new structure, and * frees up the old structure after a grace period. * * Uses rcu_assign_pointer() to ensure that concurrent readers * see the initialized version of the new structure. * * Uses call_rcu() to ensure that any readers that might have * references to the old structure complete before freeing the * old structure. */ void foo_update_a(int new_a) { struct foo *new_fp; struct foo *old_fp; new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL); spin_lock(&foo_mutex); old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex)); *new_fp = *old_fp; new_fp->a = new_a; rcu_assign_pointer(gbl_foo, new_fp); spin_unlock(&foo_mutex); call_rcu(&old_fp->rcu, foo_reclaim); } The foo_reclaim() function might appear as follows: void foo_reclaim(struct rcu_head *rp) { struct foo *fp = container_of(rp, struct foo, rcu); foo_cleanup(fp->a); kfree(fp); } The container_of() primitive is a macro that, given a pointer into a struct, the type of the struct, and the pointed-to field within the struct, returns a pointer to the beginning of the struct. The use of call_rcu() permits the caller of foo_update_a() to immediately regain control, without needing to worry further about the old version of the newly updated element. It also clearly shows the RCU distinction between updater, namely foo_update_a(), and reclaimer, namely foo_reclaim(). The summary of advice is the same as for the previous section, except that we are now using call_rcu() rather than synchronize_rcu(): o Use call_rcu() -after- removing a data element from an RCU-protected data structure in order to register a callback function that will be invoked after the completion of all RCU read-side critical sections that might be referencing that data item. If the callback for call_rcu() is not doing anything more than calling kfree() on the structure, you can use kfree_rcu() instead of call_rcu() to avoid having to write your own callback: kfree_rcu(old_fp, rcu); Again, see checklist.txt for additional rules governing the use of RCU. 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? One of the nice things about RCU is that it has extremely simple "toy" implementations that are a good first step towards understanding the production-quality implementations in the Linux kernel. This section presents two such "toy" implementations of RCU, one that is implemented in terms of familiar locking primitives, and another that more closely resembles "classic" RCU. Both are way too simple for real-world use, lacking both functionality and performance. However, they are useful in getting a feel for how RCU works. See kernel/rcupdate.c for a production-quality implementation, and see: http://www.rdrop.com/users/paulmck/RCU for papers describing the Linux kernel RCU implementation. The OLS'01 and OLS'02 papers are a good introduction, and the dissertation provides more details on the current implementation as of early 2004. 5A. "TOY" IMPLEMENTATION #1: LOCKING This section presents a "toy" RCU implementation that is based on familiar locking primitives. Its overhead makes it a non-starter for real-life use, as does its lack of scalability. It is also unsuitable for realtime use, since it allows scheduling latency to "bleed" from one read-side critical section to another. However, it is probably the easiest implementation to relate to, so is a good starting point. It is extremely simple: static DEFINE_RWLOCK(rcu_gp_mutex); void rcu_read_lock(void) { read_lock(&rcu_gp_mutex); } void rcu_read_unlock(void) { read_unlock(&rcu_gp_mutex); } void synchronize_rcu(void) { write_lock(&rcu_gp_mutex); write_unlock(&rcu_gp_mutex); } [You can ignore rcu_assign_pointer() and rcu_dereference() without missing much. But here they are anyway. And whatever you do, don't forget about them when submitting patches making use of RCU!] #define rcu_assign_pointer(p, v) ({ \ smp_wmb(); \ (p) = (v); \ }) #define rcu_dereference(p) ({ \ typeof(p) _________p1 = p; \ smp_read_barrier_depends(); \ (_________p1); \ }) The rcu_read_lock() and rcu_read_unlock() primitive read-acquire and release a global reader-writer lock. The synchronize_rcu() primitive write-acquires this same lock, then immediately releases it. This means that once synchronize_rcu() exits, all RCU read-side critical sections that were in progress before synchronize_rcu() was called are guaranteed to have completed -- there is no way that synchronize_rcu() would have been able to write-acquire the lock otherwise. It is possible to nest rcu_read_lock(), since reader-writer locks may be recursively acquired. Note also that rcu_read_lock() is immune from deadlock (an important property of RCU). The reason for this is that the only thing that can block rcu_read_lock() is a synchronize_rcu(). But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex, so there can be no deadlock cycle. Quick Quiz #1: Why is this argument naive? How could a deadlock occur when using this algorithm in a real-world Linux kernel? How could this deadlock be avoided? 5B. "TOY" EXAMPLE #2: CLASSIC RCU This section presents a "toy" RCU implementation that is based on "classic RCU". It is also short on performance (but only for updates) and on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT kernels. The definitions of rcu_dereference() and rcu_assign_pointer() are the same as those shown in the preceding section, so they are omitted. void rcu_read_lock(void) { } void rcu_read_unlock(void) { } void synchronize_rcu(void) { int cpu; for_each_possible_cpu(cpu) run_on(cpu); } Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing. This is the great strength of classic RCU in a non-preemptive kernel: read-side overhead is precisely zero, at least on non-Alpha CPUs. And there is absolutely no way that rcu_read_lock() can possibly participate in a deadlock cycle! The implementation of synchronize_rcu() simply schedules itself on each CPU in turn. The run_on() primitive can be implemented straightforwardly in terms of the sched_setaffinity() primitive. Of course, a somewhat less "toy" implementation would restore the affinity upon completion rather than just leaving all tasks running on the last CPU, but when I said "toy", I meant -toy-! So how the heck is this supposed to work??? Remember that it is illegal to block while in an RCU read-side critical section. Therefore, if a given CPU executes a context switch, we know that it must have completed all preceding RCU read-side critical sections. Once -all- CPUs have executed a context switch, then -all- preceding RCU read-side critical sections will have completed. So, suppose that we remove a data item from its structure and then invoke synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed that there are no RCU read-side critical sections holding a reference to that data item, so we can safely reclaim it. Quick Quiz #2: Give an example where Classic RCU's read-side overhead is -negative-. Quick Quiz #3: If it is illegal to block in an RCU read-side critical section, what the heck do you do in PREEMPT_RT, where normal spinlocks can block??? 6. ANALOGY WITH READER-WRITER LOCKING Although RCU can be used in many different ways, a very common use of RCU is analogous to reader-writer locking. The following unified diff shows how closely related RCU and reader-writer locking can be. @@ -13,15 +14,15 @@ struct list_head *lp; struct el *p; - read_lock(); - list_for_each_entry(p, head, lp) { + rcu_read_lock(); + list_for_each_entry_rcu(p, head, lp) { if (p->key == key) { *result = p->data; - read_unlock(); + rcu_read_unlock(); return 1; } } - read_unlock(); + rcu_read_unlock(); return 0; } @@ -29,15 +30,16 @@ { struct el *p; - write_lock(&listmutex); + spin_lock(&listmutex); list_for_each_entry(p, head, lp) { if (p->key == key) { - list_del(&p->list); - write_unlock(&listmutex); + list_del_rcu(&p->list); + spin_unlock(&listmutex); + synchronize_rcu(); kfree(p); return 1; } } - write_unlock(&listmutex); + spin_unlock(&listmutex); return 0; } Or, for those who prefer a side-by-side listing: 1 struct el { 1 struct el { 2 struct list_head list; 2 struct list_head list; 3 long key; 3 long key; 4 spinlock_t mutex; 4 spinlock_t mutex; 5 int data; 5 int data; 6 /* Other data fields */ 6 /* Other data fields */ 7 }; 7 }; 8 spinlock_t listmutex; 8 spinlock_t listmutex; 9 struct el head; 9 struct el head; 1 int search(long key, int *result) 1 int search(long key, int *result) 2 { 2 { 3 struct list_head *lp; 3 struct list_head *lp; 4 struct el *p; 4 struct el *p; 5 5 6 read_lock(); 6 rcu_read_lock(); 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) { 8 if (p->key == key) { 8 if (p->key == key) { 9 *result = p->data; 9 *result = p->data; 10 read_unlock(); 10 rcu_read_unlock(); 11 return 1; 11 return 1; 12 } 12 } 13 } 13 } 14 read_unlock(); 14 rcu_read_unlock(); 15 return 0; 15 return 0; 16 } 16 } 1 int delete(long key) 1 int delete(long key) 2 { 2 { 3 struct el *p; 3 struct el *p; 4 4 5 write_lock(&listmutex); 5 spin_lock(&listmutex); 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) { 7 if (p->key == key) { 7 if (p->key == key) { 8 list_del(&p->list); 8 list_del_rcu(&p->list); 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex); 10 synchronize_rcu(); 10 kfree(p); 11 kfree(p); 11 return 1; 12 return 1; 12 } 13 } 13 } 14 } 14 write_unlock(&listmutex); 15 spin_unlock(&listmutex); 15 return 0; 16 return 0; 16 } 17 } Either way, the differences are quite small. Read-side locking moves to rcu_read_lock() and rcu_read_unlock, update-side locking moves from a reader-writer lock to a simple spinlock, and a synchronize_rcu() precedes the kfree(). However, there is one potential catch: the read-side and update-side critical sections can now run concurrently. In many cases, this will not be a problem, but it is necessary to check carefully regardless. For example, if multiple independent list updates must be seen as a single atomic update, converting to RCU will require special care. Also, the presence of synchronize_rcu() means that the RCU version of delete() can now block. If this is a problem, there is a callback-based mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can be used in place of synchronize_rcu(). 7. FULL LIST OF RCU APIs The RCU APIs are documented in docbook-format header comments in the Linux-kernel source code, but it helps to have a full list of the APIs, since there does not appear to be a way to categorize them in docbook. Here is the list, by category. RCU list traversal: list_entry_rcu list_first_entry_rcu list_next_rcu list_for_each_entry_rcu list_for_each_entry_continue_rcu hlist_first_rcu hlist_next_rcu hlist_pprev_rcu hlist_for_each_entry_rcu hlist_for_each_entry_rcu_bh hlist_for_each_entry_continue_rcu hlist_for_each_entry_continue_rcu_bh hlist_nulls_first_rcu hlist_nulls_for_each_entry_rcu hlist_bl_first_rcu hlist_bl_for_each_entry_rcu RCU pointer/list update: rcu_assign_pointer list_add_rcu list_add_tail_rcu list_del_rcu list_replace_rcu hlist_add_behind_rcu hlist_add_before_rcu hlist_add_head_rcu hlist_del_rcu hlist_del_init_rcu hlist_replace_rcu list_splice_init_rcu() hlist_nulls_del_init_rcu hlist_nulls_del_rcu hlist_nulls_add_head_rcu hlist_bl_add_head_rcu hlist_bl_del_init_rcu hlist_bl_del_rcu hlist_bl_set_first_rcu RCU: Critical sections Grace period Barrier rcu_read_lock synchronize_net rcu_barrier rcu_read_unlock synchronize_rcu rcu_dereference synchronize_rcu_expedited rcu_read_lock_held call_rcu rcu_dereference_check kfree_rcu rcu_dereference_protected bh: Critical sections Grace period Barrier rcu_read_lock_bh call_rcu_bh rcu_barrier_bh rcu_read_unlock_bh synchronize_rcu_bh rcu_dereference_bh synchronize_rcu_bh_expedited rcu_dereference_bh_check rcu_dereference_bh_protected rcu_read_lock_bh_held sched: Critical sections Grace period Barrier rcu_read_lock_sched synchronize_sched rcu_barrier_sched rcu_read_unlock_sched call_rcu_sched [preempt_disable] synchronize_sched_expedited [and friends] rcu_read_lock_sched_notrace rcu_read_unlock_sched_notrace rcu_dereference_sched rcu_dereference_sched_check rcu_dereference_sched_protected rcu_read_lock_sched_held SRCU: Critical sections Grace period Barrier srcu_read_lock synchronize_srcu srcu_barrier srcu_read_unlock call_srcu srcu_dereference synchronize_srcu_expedited srcu_dereference_check srcu_read_lock_held SRCU: Initialization/cleanup init_srcu_struct cleanup_srcu_struct All: lockdep-checked RCU-protected pointer access rcu_access_pointer rcu_dereference_raw RCU_LOCKDEP_WARN rcu_sleep_check RCU_NONIDLE See the comment headers in the source code (or the docbook generated from them) for more information. However, given that there are no fewer than four families of RCU APIs in the Linux kernel, how do you choose which one to use? The following list can be helpful: a. Will readers need to block? If so, you need SRCU. b. What about the -rt patchset? If readers would need to block in an non-rt kernel, you need SRCU. If readers would block in a -rt kernel, but not in a non-rt kernel, SRCU is not necessary. c. Do you need to treat NMI handlers, hardirq handlers, and code segments with preemption disabled (whether via preempt_disable(), local_irq_save(), local_bh_disable(), or some other mechanism) as if they were explicit RCU readers? If so, RCU-sched is the only choice that will work for you. d. Do you need RCU grace periods to complete even in the face of softirq monopolization of one or more of the CPUs? For example, is your code subject to network-based denial-of-service attacks? If so, you need RCU-bh. e. Is your workload too update-intensive for normal use of RCU, but inappropriate for other synchronization mechanisms? If so, consider SLAB_DESTROY_BY_RCU. But please be careful! f. Do you need read-side critical sections that are respected even though they are in the middle of the idle loop, during user-mode execution, or on an offlined CPU? If so, SRCU is the only choice that will work for you. g. Otherwise, use RCU. Of course, this all assumes that you have determined that RCU is in fact the right tool for your job. 8. ANSWERS TO QUICK QUIZZES Quick Quiz #1: Why is this argument naive? How could a deadlock occur when using this algorithm in a real-world Linux kernel? [Referring to the lock-based "toy" RCU algorithm.] Answer: Consider the following sequence of events: 1. CPU 0 acquires some unrelated lock, call it "problematic_lock", disabling irq via spin_lock_irqsave(). 2. CPU 1 enters synchronize_rcu(), write-acquiring rcu_gp_mutex. 3. CPU 0 enters rcu_read_lock(), but must wait because CPU 1 holds rcu_gp_mutex. 4. CPU 1 is interrupted, and the irq handler attempts to acquire problematic_lock. The system is now deadlocked. One way to avoid this deadlock is to use an approach like that of CONFIG_PREEMPT_RT, where all normal spinlocks become blocking locks, and all irq handlers execute in the context of special tasks. In this case, in step 4 above, the irq handler would block, allowing CPU 1 to release rcu_gp_mutex, avoiding the deadlock. Even in the absence of deadlock, this RCU implementation allows latency to "bleed" from readers to other readers through synchronize_rcu(). To see this, consider task A in an RCU read-side critical section (thus read-holding rcu_gp_mutex), task B blocked attempting to write-acquire rcu_gp_mutex, and task C blocked in rcu_read_lock() attempting to read_acquire rcu_gp_mutex. Task A's RCU read-side latency is holding up task C, albeit indirectly via task B. Realtime RCU implementations therefore use a counter-based approach where tasks in RCU read-side critical sections cannot be blocked by tasks executing synchronize_rcu(). Quick Quiz #2: Give an example where Classic RCU's read-side overhead is -negative-. Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT kernel where a routing table is used by process-context code, but can be updated by irq-context code (for example, by an "ICMP REDIRECT" packet). The usual way of handling this would be to have the process-context code disable interrupts while searching the routing table. Use of RCU allows such interrupt-disabling to be dispensed with. Thus, without RCU, you pay the cost of disabling interrupts, and with RCU you don't. One can argue that the overhead of RCU in this case is negative with respect to the single-CPU interrupt-disabling approach. Others might argue that the overhead of RCU is merely zero, and that replacing the positive overhead of the interrupt-disabling scheme with the zero-overhead RCU scheme does not constitute negative overhead. In real life, of course, things are more complex. But even the theoretical possibility of negative overhead for a synchronization primitive is a bit unexpected. ;-) Quick Quiz #3: If it is illegal to block in an RCU read-side critical section, what the heck do you do in PREEMPT_RT, where normal spinlocks can block??? Answer: Just as PREEMPT_RT permits preemption of spinlock critical sections, it permits preemption of RCU read-side critical sections. It also permits spinlocks blocking while in RCU read-side critical sections. Why the apparent inconsistency? Because it is it possible to use priority boosting to keep the RCU grace periods short if need be (for example, if running short of memory). In contrast, if blocking waiting for (say) network reception, there is no way to know what should be boosted. Especially given that the process we need to boost might well be a human being who just went out for a pizza or something. And although a computer-operated cattle prod might arouse serious interest, it might also provoke serious objections. Besides, how does the computer know what pizza parlor the human being went to??? ACKNOWLEDGEMENTS My thanks to the people who helped make this human-readable, including Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern. For more information, see http://www.rdrop.com/users/paulmck/RCU. */ Using RCU to Protect Read-Mostly Linked Lists One of the best applications of RCU is to protect read-mostly linked lists ("struct list_head" in list.h). One big advantage of this approach is that all of the required memory barriers are included for you in the list macros. This document describes several applications of RCU, with the best fits first. Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates The best applications are cases where, if reader-writer locking were used, the read-side lock would be dropped before taking any action based on the results of the search. The most celebrated example is the routing table. Because the routing table is tracking the state of equipment outside of the computer, it will at times contain stale data. Therefore, once the route has been computed, there is no need to hold the routing table static during transmission of the packet. After all, you can hold the routing table static all you want, but that won't keep the external Internet from changing, and it is the state of the external Internet that really matters. In addition, routing entries are typically added or deleted, rather than being modified in place. A straightforward example of this use of RCU may be found in the system-call auditing support. For example, a reader-writer locked implementation of audit_filter_task() might be as follows: static enum audit_state audit_filter_task(struct task_struct *tsk) { struct audit_entry *e; enum audit_state state; read_lock(&auditsc_lock); /* Note: audit_netlink_sem held by caller. */ list_for_each_entry(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { read_unlock(&auditsc_lock); return state; } } read_unlock(&auditsc_lock); return AUDIT_BUILD_CONTEXT; } Here the list is searched under the lock, but the lock is dropped before the corresponding value is returned. By the time that this value is acted on, the list may well have been modified. This makes sense, since if you are turning auditing off, it is OK to audit a few extra system calls. This means that RCU can be easily applied to the read side, as follows: static enum audit_state audit_filter_task(struct task_struct *tsk) { struct audit_entry *e; enum audit_state state; rcu_read_lock(); /* Note: audit_netlink_sem held by caller. */ list_for_each_entry_rcu(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { rcu_read_unlock(); return state; } } rcu_read_unlock(); return AUDIT_BUILD_CONTEXT; } The read_lock() and read_unlock() calls have become rcu_read_lock() and rcu_read_unlock(), respectively, and the list_for_each_entry() has become list_for_each_entry_rcu(). The _rcu() list-traversal primitives insert the read-side memory barriers that are required on DEC Alpha CPUs. The changes to the update side are also straightforward. A reader-writer lock might be used as follows for deletion and insertion: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) { struct audit_entry *e; write_lock(&auditsc_lock); list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { list_del(&e->list); write_unlock(&auditsc_lock); return 0; } } write_unlock(&auditsc_lock); return -EFAULT; /* No matching rule */ } static inline int audit_add_rule(struct audit_entry *entry, struct list_head *list) { write_lock(&auditsc_lock); if (entry->rule.flags & AUDIT_PREPEND) { entry->rule.flags &= ~AUDIT_PREPEND; list_add(&entry->list, list); } else { list_add_tail(&entry->list, list); } write_unlock(&auditsc_lock); return 0; } Following are the RCU equivalents for these two functions: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) { struct audit_entry *e; /* Do not use the _rcu iterator here, since this is the only * deletion routine. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { list_del_rcu(&e->list); call_rcu(&e->rcu, audit_free_rule); return 0; } } return -EFAULT; /* No matching rule */ } static inline int audit_add_rule(struct audit_entry *entry, struct list_head *list) { if (entry->rule.flags & AUDIT_PREPEND) { entry->rule.flags &= ~AUDIT_PREPEND; list_add_rcu(&entry->list, list); } else { list_add_tail_rcu(&entry->list, list); } return 0; } Normally, the write_lock() and write_unlock() would be replaced by a spin_lock() and a spin_unlock(), but in this case, all callers hold audit_netlink_sem, so no additional locking is required. The auditsc_lock can therefore be eliminated, since use of RCU eliminates the need for writers to exclude readers. Normally, the write_lock() calls would be converted into spin_lock() calls. The list_del(), list_add(), and list_add_tail() primitives have been replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu(). The _rcu() list-manipulation primitives add memory barriers that are needed on weakly ordered CPUs (most of them!). The list_del_rcu() primitive omits the pointer poisoning debug-assist code that would otherwise cause concurrent readers to fail spectacularly. So, when readers can tolerate stale data and when entries are either added or deleted, without in-place modification, it is very easy to use RCU! Example 2: Handling In-Place Updates The system-call auditing code does not update auditing rules in place. However, if it did, reader-writer-locked code to do so might look as follows (presumably, the field_count is only permitted to decrease, otherwise, the added fields would need to be filled in): static inline int audit_upd_rule(struct audit_rule *rule, struct list_head *list, __u32 newaction, __u32 newfield_count) { struct audit_entry *e; struct audit_newentry *ne; write_lock(&auditsc_lock); /* Note: audit_netlink_sem held by caller. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { e->rule.action = newaction; e->rule.file_count = newfield_count; write_unlock(&auditsc_lock); return 0; } } write_unlock(&auditsc_lock); return -EFAULT; /* No matching rule */ } The RCU version creates a copy, updates the copy, then replaces the old entry with the newly updated entry. This sequence of actions, allowing concurrent reads while doing a copy to perform an update, is what gives RCU ("read-copy update") its name. The RCU code is as follows: static inline int audit_upd_rule(struct audit_rule *rule, struct list_head *list, __u32 newaction, __u32 newfield_count) { struct audit_entry *e; struct audit_newentry *ne; list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { ne = kmalloc(sizeof(*entry), GFP_ATOMIC); if (ne == NULL) return -ENOMEM; audit_copy_rule(&ne->rule, &e->rule); ne->rule.action = newaction; ne->rule.file_count = newfield_count; list_replace_rcu(&e->list, &ne->list); call_rcu(&e->rcu, audit_free_rule); return 0; } } return -EFAULT; /* No matching rule */ } Again, this assumes that the caller holds audit_netlink_sem. Normally, the reader-writer lock would become a spinlock in this sort of code. Example 3: Eliminating Stale Data The auditing examples above tolerate stale data, as do most algorithms that are tracking external state. Because there is a delay from the time the external state changes before Linux becomes aware of the change, additional RCU-induced staleness is normally not a problem. However, there are many examples where stale data cannot be tolerated. One example in the Linux kernel is the System V IPC (see the ipc_lock() function in ipc/util.c). This code checks a "deleted" flag under a per-entry spinlock, and, if the "deleted" flag is set, pretends that the entry does not exist. For this to be helpful, the search function must return holding the per-entry spinlock, as ipc_lock() does in fact do. Quick Quiz: Why does the search function need to return holding the per-entry lock for this deleted-flag technique to be helpful? If the system-call audit module were to ever need to reject stale data, one way to accomplish this would be to add a "deleted" flag and a "lock" spinlock to the audit_entry structure, and modify audit_filter_task() as follows: static enum audit_state audit_filter_task(struct task_struct *tsk) { struct audit_entry *e; enum audit_state state; rcu_read_lock(); list_for_each_entry_rcu(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { spin_lock(&e->lock); if (e->deleted) { spin_unlock(&e->lock); rcu_read_unlock(); return AUDIT_BUILD_CONTEXT; } rcu_read_unlock(); return state; } } rcu_read_unlock(); return AUDIT_BUILD_CONTEXT; } Note that this example assumes that entries are only added and deleted. Additional mechanism is required to deal correctly with the update-in-place performed by audit_upd_rule(). For one thing, audit_upd_rule() would need additional memory barriers to ensure that the list_add_rcu() was really executed before the list_del_rcu(). The audit_del_rule() function would need to set the "deleted" flag under the spinlock as follows: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) { struct audit_entry *e; /* Do not need to use the _rcu iterator here, since this * is the only deletion routine. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { spin_lock(&e->lock); list_del_rcu(&e->list); e->deleted = 1; spin_unlock(&e->lock); call_rcu(&e->rcu, audit_free_rule); return 0; } } return -EFAULT; /* No matching rule */ } Summary Read-mostly list-based data structures that can tolerate stale data are the most amenable to use of RCU. The simplest case is where entries are either added or deleted from the data structure (or atomically modified in place), but non-atomic in-place modifications can be handled by making a copy, updating the copy, then replacing the original with the copy. If stale data cannot be tolerated, then a "deleted" flag may be used in conjunction with a per-entry spinlock in order to allow the search function to reject newly deleted data. Answer to Quick Quiz Why does the search function need to return holding the per-entry lock for this deleted-flag technique to be helpful? If the search function drops the per-entry lock before returning, then the caller will be processing stale data in any case. If it is really OK to be processing stale data, then you don't need a "deleted" flag. If processing stale data really is a problem, then you need to hold the per-entry lock across all of the code that uses the value that was returned.
/*
* 创建一个新的结构 foo，该结构与 gbl_foo 当前指向的结构 foo 相同，
* 只不过字段 “a” 被 “new_a” 替换。将 gbl_foo 指向新结构，
* 并在宽限期过后释放旧结构。
*
* 使用 rcu_assign_pointer() 确保并发读者看到新结构初始化过的版本。
*
* 使用 syncnize_rcu() 来确保在释放旧结构之前，
* 所有可能引用旧结构的读者都已完成。
*/
void foo_update_a(int new_a)
{
struct foo *new_fp;
struct foo *old_fp;
new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
spin_lock(&foo_mutex);
old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
*new_fp = *old_fp;
new_fp->a = new_a;
rcu_assign_pointer(gbl_foo, new_fp);
spin_unlock(&foo_mutex);
synchronize_rcu();
kfree(old_fp);
}
/*
* 返回当前 gbl_foo 结构字段 “a” 的值。
* 使用 rcu_read_lock() 和 rcu_read_unlock()
* 确保该结构不会从他们之间删除删除，并使用 rcu_dereference()
* 确保我们看到该结构的初始化过版本
* （对于 DEC Alpha 和阅读代码的人来说很重要）。
*/
int foo_get_a(void)
{
int retval;
rcu_read_lock();
retval = rcu_dereference(gbl_foo)->a;
rcu_read_unlock();
return retval;
}- 使用 rcu_read_lock() 和 rcu_read_unlock() 来保护 RCU 读端临界区。
- 在 RCU 读端临界区中，使用 rcu_dereference() 去间接访问受 RCU 保护的指针。
- 使用一些可靠的方案（例如锁或信号量）可以防止并发更新相互干扰。
- 使用 rcu_assign_pointer() 更新受 RCU 保护的指针。这个原语保护并发读者免受更新程序的干扰，而不是彼此并发更新！ 因此，你仍然需要使用锁定（或类似方法）来防止并发的 rcu_assign_pointer() 原语相互干扰。
- 在从受RCU保护的数据结构中删除数据元素之后，但在回收/释放数据元素之前，请使用 syncnize_rcu() ，以便等待所有可能引用该数据项的 RCU 读端临界区的完成。
在使用RCU时，对共享资源的访问在大部分时间应该是只读的，写访问应该相对较少，因为写访问多了必然相对于其他锁机制而已更占系统资源，影响效率。其次是读者在持有rcu_read_lock（RCU读锁定函数）的时候，不能发生进程上下文切换，否则，因为写者需要等待读者完成方可进行，则此时写者进程也会一直被阻塞，影响系统的正常运行。再次写者执行完毕后需要调用回调函数，此时发生上下文切换，当前进程进入睡眠，则系统将一直不能调用回调函数，更槽糕的是，此时其它进程若再去执行共享的临界区，必然造成一定的错误。最后一点是受RCU机制保护的资源必须是通过指针访问。因为从RCU机制上看，几乎所有操作都是针对指针数据的;
同步函数最为重要，即synchronize_rcu()。读者函数的实质其实很简单：禁止抢占，也就是说在RCU期间不允许发生进程上下文切换，原因上述已提及，即是写者需要等待读者完成方可进行，则此时写者进程也会一直被阻塞，影响系统的正常运行等，故而不允许在RCU期间发生进程上下文切换
关于写者函数，主要就是call_rcu和call_rcu_bh两个函数。其中call_rcu能实现的功能是它不会使写者阻塞，因而它可在中断上下文及软中断使用，该函数将函数func挂接到RCU的回调函数链表上，然后立即返回，读者函数中提及的synchronize_rcu()函数在实现时也调用了该函数。而call_rcu_bh函数实现的功能几乎与call_rcu完全相同，唯一的差别是它将软中断的完成当作经历一个quiescent state（静默状态，本节一开始有提及这个概念）， 因此若写者使用了该函数，那么读者需对应的使用rcu_read_lock_bh() 和rcu_read_unlock_bh()。
· 使用rcu_read_lock_bh() 和rcu_read_unlock_bh()函数的原因是由于call_rcu_bh函数不会使写者阻塞，可在中断上下文及软中断使用。这表明此时系统中的中断和软中断并没有被关闭。那么写者在调用call_rcu_bh函数访问临界区时，RCU机制下的读者也能访问临界区。此时对于读者而言，它若是需要读取临界区的内容，它必须把软中断关闭，以免读者在当前的进程上下文过程中被软中断打断（上述内容提过软中断可以打断当前的进程上下文）。而rcu_read_lock_bh() 和rcu_read_unlock_bh()函数的实质是调用local_bh_disable()和local_bh_enable()函数，显然这是实现了禁止软中断和使能软中断的功能。
另外在Linux源码中关于call_rcu_bh函数的注释中还明确说明了如果当前的进程是在中断上下文中，则需要执行rcu_read_lock()和rcu_read_unlock()，结合这两个函数的实现实质表明它实际上禁止或使能内核的抢占调度，原因不言而喻，避免当前进程在执行读写过程中被其它进程抢占。同时内核注释还表明call_rcu_bh这个接口函数的使用条件是在大部分的读临界区操作发生在软中断上下文中，原因还是需从它实现的功能出发，相信很容易理解，主要是要从执行效率方面考虑。
static inline void rcu_read_lock_bh(void); static inline void rcu_read_unlock_bh(void);
这个变种只在修改是通过 call_rcu_bh进行的情况下使用，因为 call_rcu_bh将把 softirq 的执行完毕也认为是一个 quiescent state，因此如果修改是通过 call_rcu_bh 进行的，在进程上下文的读端临界区必须使用这一变种
每一个 CPU 维护两个数据结构 rcu_sched_data，rcu_bh_data，它们用于保存回调函数。函数call_rcu和函数call_rcu_bh用于注册回调函数，前者把回调函数注册到rcu_sched_data，而后者则把回调函数注册到rcu_bh_data，在每一个数据结构上，回调函数被组成一个链表，先注册的排在前头，后注册的排在末尾；时钟中断处理函数（update_process_times）调用函数rcu_check_callbacks
函数rcu_check_callbacks首先检查该CPU是否经历了一个quiescent state，如果(或)：
- 当前进程运行在用户态；
- 当前进程为idle且当前不处在运行softirq状态，也不处在运行IRQ处理函数的状态；
该CPU已经经历了一个quiescent state，因此通过调用函数rcu_sched_qs和rcu_bh_qs标记该CPU的数据结构rcu_sched_data和rcu_bh_data的标记字段passed_quiesc，以记录该CPU已经经历一个quiescent state。
否则，如果当前不处在运行softirq状态，那么，只标记该CPU的数据结构rcu_bh_data的标记字段passed_quiesc，以记录该CPU已经经历一个quiescent state。注意，该标记只对rcu_bh_data有效。
然后，函数rcu_check_callbacks将调用开启RCU_SOFTIRQ。
synchronize_rcu()在RCU中是一个最核心的函数,它用来等待之前的读者全部退出。
在完整的宽限期结束后，即在所有当前正在执行的RCU读取端临界区完成之后，控制权会在一段时间后返回给调用者。
https://www.cnblogs.com/alantu2018/p/8459359.html
1．经典RCU 如果不关心使用的RCU是不可抢占RCU还是可抢占RCU，应该使用经典RCU的编程接口。最初的经典RCU是不可抢占RCU，后来实现了可抢占RCU，经典RCU的意思发生了变化：如果内核编译了可抢占RCU，那么经典RCU的编程接口被实现为可抢占RCU，否则被实现为不可抢占RCU。 读者使用函数rcu_read_lock()标记进入读端临界区，使用函数rcu_read_unlock()标记退出读端临界区。读端临界区可以嵌套。 在读端临界区里面应该使用宏rcu_dereference(p)访问指针，这个宏封装了数据依赖屏障，即只有阿尔法处理器需要的读内存屏障。 写者可以使用下面4个函数。 （1）使用函数synchronize_rcu()等待宽限期结束，即所有读者退出读端临界区，然后写者执行下一步操作。这个函数可能睡眠。 （2）使用函数synchronize_rcu_expedited()等待宽限期结束。和函数synchronize_rcu()的区别是：该函数会向其他处理器发送处理器间中断（Inter-Processor Interrupt，IPI）请求，强制宽限期快速结束。我们把强制快速结束的宽限期称为加速宽限期（expedited grace period），把没有强制快速结束的宽限期称为正常宽限期（normal grace period）。 （3）使用函数call_rcu()注册延后执行的回调函数，把回调函数添加到RCU回调函数链表中，立即返回，不会阻塞。函数原型如下： void call_rcu(struct rcu_head *head, rcu_callback_t func); struct callback_head { struct callback_head *next; void (*func)(struct callback_head *head); } __attribute__((aligned(sizeof(void *)))); #define rcu_head callback_head typedef void (*rcu_callback_t)(struct rcu_head *head); （4）使用函数rcu_barrier()等待使用call_rcu注册的所有回调函数执行完。这个函数可能睡眠。 现在举例说明使用方法，假设链表节点和头节点如下： typedef struct { struct list_head link; struct rcu_head rcu; int key; int val; } test_entry; struct list_head test_head; 成员“struct rcu_head rcu”：调用函数call_rcu把回调函数添加到RCU回调函数链表的时候需要使用。 读者访问链表的方法如下： int test_read(int key, int *val_ptr) { test_entry *entry; int found = 0; rcu_read_lock(); list_for_each_entry_rcu(entry, &test_head, link) { if (entry->key == key) { *val_ptr = entry->val; found = 1; break; } } rcu_read_unlock(); return found; } 如果只有一个写者，写者不需要使用锁，添加、更新和删除3种操作的实现方法如下。 （1）写者添加一个节点到链表尾部。 void test_add_node(test_entry *entry) { list_add_tail_rcu(&entry->link, &test_head); } （2）写者更新一个节点。 更新的过程是：首先把旧节点复制更新，然后使用新节点替换旧节点，最后使用函数call_rcu注册回调函数，延后释放旧节点。 int test_update_node(int key, int new_val) { test_entry *entry, *new_entry; int ret; ret = -ENOENT; list_for_each_entry(entry, &test_head, link) { if (entry->key == key) { new_entry = kmalloc(sizeof(test_entry), GFP_ATOMIC); if (new_entry == NULL) { ret = -ENOMEM; break; } *new_entry = *entry; new_entry->val = new_val; list_replace_rcu(&entry->list, &new_entry->list); call_rcu(&entry->rcu, test_free_node); ret = 0; break; } } return ret; } static void test_free_node(struct rcu_head *head) { test_entry *entry = container_of(head, test_entry, rcu); kfree(entry); } （3）写者删除一个节点的第一种实现方法是：首先把节点从链表中删除，然后使用函数call_rcu注册回调函数，延后释放节点。 int test_del_node(int key) { test_entry *entry; int found = 0; list_for_each_entry(entry, &test_head, link) { if (entry->key == key) { list_del_rcu(&entry->link); call_rcu(&entry->rcu, test_free_node); found = 1; break; } } return found; } static void test_free_node(struct rcu_head *head) { test_entry *entry = container_of(head, test_entry, rcu); kfree(entry); } （4）写者删除一个节点的第二种实现方法是：首先把节点从链表中删除，然后使用函数synchronize_rcu()等待宽限期结束，最后释放节点。 int test_del_node(int key) { test_entry *entry; int found = 0; list_for_each_entry(entry, &test_head, link) { if (entry->key == key) { list_del_rcu(&entry->link); synchronize_rcu(); kfree(entry); found = 1; break; } } return found; } 如果有多个写者，那么写者之间必须使用锁互斥，添加、更新和删除3种操作的实现方法如下。 （1）写者添加一个节点到链表尾部。假设使用自旋锁“test_lock”保护链表。 void test_add_node(test_entry *entry) { spin_lock(&test_lock); list_add_tail_rcu(&entry->link, &test_head); spin_unlock(&test_lock); } （2）写者更新一个节点。 int test_update_node(int key, int new_val) { test_entry *entry, *new_entry; int ret; ret = -ENOENT; spin_lock(&test_lock); list_for_each_entry(entry, &test_head, link) { if (entry->key == key) { new_entry = kmalloc(sizeof(test_entry), GFP_ATOMIC); if (new_entry == NULL) { ret = -ENOMEM; break; } *new_entry = *entry; new_entry->val = new_val; list_replace_rcu(&entry->list, &new_entry->list); call_rcu(&entry->rcu, test_free_node); ret = 0; break; } } spin_unlock(&test_lock); return ret; } static void test_free_node(struct rcu_head *head) { test_entry *entry = container_of(head, test_entry, rcu); kfree(entry); } （3）写者删除一个节点。 int test_del_node(int key) { test_entry *entry； int found = 0; spin_lock(&test_lock); list_for_each_entry(entry, &test_head, link) { if (entry->key == key) { list_del_rcu(&entry->link); call_rcu(&entry->rcu, test_free_node); found = 1; break; } } spin_unlock(&test_lock); return found; } static void test_free_node(struct rcu_head *head) { test_entry *entry = container_of(head, test_entry, rcu); kfree(entry); } 2．不可抢占RCU 如果我们的需求是“不管内核是否编译了可抢占RCU，都要使用不可抢占RCU”，那么应该使用不可抢占RCU的专用编程接口。 读者使用函数rcu_read_lock_sched()标记进入读端临界区，使用函数rcu_read_unlock_ sched()标记退出读端临界区。读端临界区可以嵌套。 在读端临界区里面应该使用宏rcu_dereference_sched(p)访问指针，这个宏封装了数据依赖屏障，即只有阿尔法处理器需要的读内存屏障。 写者可以使用下面4个函数。 （1）使用函数synchronize_sched()等待宽限期结束，即所有读者退出读端临界区，然后写者执行下一步操作。这个函数可能睡眠。 （2）使用函数synchronize_sched_expedited()等待宽限期结束。和函数synchronize_sched()的区别是：该函数会向其他处理器发送处理器间中断请求，强制宽限期快速结束。 （3）使用函数call_rcu_sched()注册延后执行的回调函数，把回调函数添加到RCU回调函数链表中，立即返回，不会阻塞。 （4）使用函数rcu_barrier_sched()等待使用call_rcu_sched注册的所有回调函数执行完。这个函数可能睡眠。 3．加速版不可抢占RCU 加速版不可抢占RCU在软中断很多的情况下可以缩短宽限期。 读者使用函数rcu_read_lock_bh()标记进入读端临界区，使用函数rcu_read_unlock_bh()标记退出读端临界区。读端临界区可以嵌套。 在读端临界区里面应该使用宏rcu_dereference_bh(p)访问指针，这个宏封装了数据依赖屏障，即只有阿尔法处理器需要的读内存屏障。 写者可以使用下面4个函数。 （1）使用函数synchronize_rcu_bh()等待宽限期结束，即所有读者退出读端临界区，然后写者执行下一步操作。这个函数可能睡眠。 （2）使用函数synchronize_rcu_bh_expedited()等待宽限期结束。和函数synchronize_rcu_ bh()的区别是：该函数会向其他处理器发送处理器间中断请求，强制宽限期快速结束。 （3）使用函数call_rcu_bh注册延后执行的回调函数，把回调函数添加到RCU回调函数链表中，立即返回，不会阻塞。 （4）使用函数rcu_barrier_bh()等待使用call_rcu_bh注册的所有回调函数执行完。这个函数可能睡眠。
之前一直好奇rcu_dereference() vs rcu_dereference_protected()? 见https://stackoverflow.com/questions/39251287/rcu-dereference-vs-rcu-dereference-protected
In short:
rcu_dereference()should be used at read-side, protected byrcu_read_lock()or similar.rcu_dereference_protected()should be used at write-side (update-side) by the single writer, or protected by the lock which prevents several writers from concurrent modification of the dereferenced pointer. In such cases pointer cannot be modified outside of the current thread, so neither compiler- nor cpu-barriers are needed.
If doubt, using rcu_dereference is always safe, and its perfomance penalties (compared to rcu_dereference_protected) are low.
Return the value of the specified RCU-protected pointer, but omit
* both the smp_read_barrier_depends() and the READ_ONCE(). This
* is useful in cases where update-side locks prevent the value of the
* pointer from changing. Please note that this primitive does -not-
* prevent the compiler from repeating this reference or combining it
* with other references, so it should not be used without protection
* of appropriate locks.
*
* This function is only for update-side use. Using this function
* when protected only by rcu_read_lock() will result in infrequent
* but very ugly failures.
返回指定的 RCU 保护指针的值，但省略 smp_read_barrier_depends() 和 READ_ONCE()。这在更新端锁防止指针值更改的情况下很有用。请注意，这个原语不会阻止编译器重复此引用或将其与其他引用合并，因此在没有适当锁的保护下不应使用它。
此函数仅用于更新端。如果在仅由 rcu_read_lock() 保护的情况下使用此函数，将会导致不频繁但非常严重的故障。 *应由单个写入器在写端中使用，或者由锁保护，以防止多个写入器并发修改取消引用的指针。
*在这种情况下，指针不能在当前线程之外进行修改，因此不需要编译器--也不需要cpu--屏障。
-
临界区要求：
rcu_dereference()：需要在 RCU 读侧临界区内。rcu_dereference_protected()：不需要在 RCU 读侧临界区内，但需要有保证安全的条件。
-
安全条件：
rcu_dereference()：安全性由 RCU 读侧临界区保证。rcu_dereference_protected()：安全性由指定的条件（通常是持有锁）保证。
-
使用场景：
rcu_dereference()：当你在 RCU 读侧临界区内，需要访问 RCU 保护的指针时使用。rcu_dereference_protected()：当你不在 RCU 读侧临界区，但有其他条件（如持有锁）确保解引用安全时使用。
- 读者需要阻塞吗？ 如果是需要，你要用 SRCU。
- 你是否需要 NMI 处理程序，硬件中断处理程序禁用了抢占功能的代码段 （无论是通过 preempt_disable(), local_irq_save(), local_bh_disable() 还是其他某种机制）如果是这样，RCU 调度rcu_read_lock_sched是唯一适合你的选择。
- 面对由软中断独占的一个或多个CPU，你是否也需要在 RCU 宽限期来完成？ 例如，你的代码是否遭受基于网络的拒绝服务攻击？ 如果是这样，则应在所有读者之间禁用 softirq，例如，通过使用 rcu_read_lock_bh()。
- 对于正常使用 RCU，你的工作量是否更新密集，但对于其他同步机制却不合适？ 如果是这样，请考虑 SLAB_TYPESAFE_BY_RCU（最初名为SLAB_DESTROY_BY_RCU）。 但是请小心！
- 你是否要在空闲循环的中间，在用户模式执行期间或在脱机的CPU上也需要读端临界区？ 如果是这样，SRCU是唯一适合你的选择。
- 除此之外用 RCU.
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--身高体重180的胖子
