Java魔法类之Unsafe(park与unpark)底层实现
一、JVM层
Unsafe.park和Unsafe.unpark是sun.misc.Unsafe类的native方法，
public native void unpark(Object var1);
public native void park(boolean var1, long var2);
这两个方法的实现是在JVM的hotspot/src/share/vm/prims/unsafe.cpp文件中，
UNSAFE_ENTRY(void, Unsafe_Park(JNIEnv *env, jobject unsafe, jboolean isAbsolute, jlong time))
UnsafeWrapper("Unsafe_Park");
EventThreadPark event;
#ifndef USDT2
HS_DTRACE_PROBE3(hotspot, thread__park__begin, thread->parker(), (int) isAbsolute, time);
#else /* USDT2 */
HOTSPOT_THREAD_PARK_BEGIN(
(uintptr_t) thread->parker(), (int) isAbsolute, time);
#endif /* USDT2 */
JavaThreadParkedState jtps(thread, time != 0);
thread->parker()->park(isAbsolute != 0, time);
#ifndef USDT2
HS_DTRACE_PROBE1(hotspot, thread__park__end, thread->parker());
#else /* USDT2 */
HOTSPOT_THREAD_PARK_END(
(uintptr_t) thread->parker());
#endif /* USDT2 */
if (event.should_commit()) {
const oop obj = thread->current_park_blocker();
if (time == 0) {
post_thread_park_event(&event, obj, min_jlong, min_jlong);
} else {
if (isAbsolute != 0) {
post_thread_park_event(&event, obj, min_jlong, time);
} else {
post_thread_park_event(&event, obj, time, min_jlong);
}
}
}
UNSAFE_END
核心是逻辑是thread->parker()->park(isAbsolute != 0, time);就是获取java线程的parker对象，然后执行它的park方法。每个java线程都有一个Parker实例，Parker类是这样定义的：
class Parker : public os::PlatformParker {
private:
volatile int _counter ;
//...
public:
void park(bool isAbsolute, jlong time);
void unpark();
//...
}
class PlatformParker : public CHeapObj<mtInternal> {
protected:
enum {
REL_INDEX = 0,
ABS_INDEX = 1
};
int _cur_index; // which cond is in use: -1, 0, 1
pthread_mutex_t _mutex [1] ;
pthread_cond_t _cond [2] ; // one for relative times and one for abs.
public: // TODO-FIXME: make dtor private
~PlatformParker() { guarantee (0, "invariant") ; }
public:
PlatformParker() {
int status;
status = pthread_cond_init (&_cond[REL_INDEX], os::Linux::condAttr());
assert_status(status == 0, status, "cond_init rel");
status = pthread_cond_init (&_cond[ABS_INDEX], NULL);
assert_status(status == 0, status, "cond_init abs");
status = pthread_mutex_init (_mutex, NULL);
assert_status(status == 0, status, "mutex_init");
_cur_index = -1; // mark as unused
}
};
park方法：
void Parker::park(bool isAbsolute, jlong time) {
// Return immediately if a permit is available.
// We depend on Atomic::xchg() having full barrier semantics
// since we are doing a lock-free update to _counter.
if (Atomic::xchg(0, &_counter) > 0) return;
Thread* thread = Thread::current();
assert(thread->is_Java_thread(), "Must be JavaThread");
JavaThread *jt = (JavaThread *)thread;
if (Thread::is_interrupted(thread, false)) {
return;
}
// Next, demultiplex/decode time arguments
timespec absTime;
if (time < 0 || (isAbsolute && time == 0) ) { // don't wait at all
return;
}
if (time > 0) {
unpackTime(&absTime, isAbsolute, time);
}
////进入safepoint region，更改线程为阻塞状态
ThreadBlockInVM tbivm(jt);
// Don't wait if cannot get lock since interference arises from
// unblocking. Also. check interrupt before trying wait
if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
//如果线程被中断，或者尝试给互斥变量加锁时失败，比如被其它线程锁住了，直接返回
return;
}
//到这里，意味着pthread_mutex_trylock(_mutex)成功
int status ;
if (_counter > 0) { // no wait needed
_counter = 0;
status = pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
OrderAccess::fence();
return;
}
#ifdef ASSERT
// Don't catch signals while blocked; let the running threads have the signals.
// (This allows a debugger to break into the running thread.)
sigset_t oldsigs;
sigset_t* allowdebug_blocked = os::Linux::allowdebug_blocked_signals();
pthread_sigmask(SIG_BLOCK, allowdebug_blocked, &oldsigs);
#endif
OSThreadWaitState osts(thread->osthread(), false /* not Object.wait() */);
jt->set_suspend_equivalent();
// cleared by handle_special_suspend_equivalent_condition() or java_suspend_self()
assert(_cur_index == -1, "invariant");
if (time == 0) {
_cur_index = REL_INDEX; // arbitrary choice when not timed
status = pthread_cond_wait (&_cond[_cur_index], _mutex) ;
} else {
_cur_index = isAbsolute ? ABS_INDEX : REL_INDEX;
status = os::Linux::safe_cond_timedwait (&_cond[_cur_index], _mutex, &absTime) ;
if (status != 0 && WorkAroundNPTLTimedWaitHang) {
pthread_cond_destroy (&_cond[_cur_index]) ;
pthread_cond_init (&_cond[_cur_index], isAbsolute ? NULL : os::Linux::condAttr());
}
}
_cur_index = -1;
assert_status(status == 0 || status == EINTR ||
status == ETIME || status == ETIMEDOUT,
status, "cond_timedwait");
#ifdef ASSERT
pthread_sigmask(SIG_SETMASK, &oldsigs, NULL);
#endif
_counter = 0 ;
status = pthread_mutex_unlock(_mutex) ;
assert_status(status == 0, status, "invariant") ;
// Paranoia to ensure our locked and lock-free paths interact
// correctly with each other and Java-level accesses.
OrderAccess::fence();
// If externally suspended while waiting, re-suspend
if (jt->handle_special_suspend_equivalent_condition()) {
jt->java_suspend_self();
}
}
park的思路：parker内部有个关键字段_counter，这个counter用来记录所谓的“permit”，当_counter大于0时，意味着有permit，然后就可以把_counter设置为0，就算是获得了permit，可以继续运行后面的代码。如果此时_counter不大于0，则等待这个条件满足。
下面我具体来看看park的具体实现：
- 当调用
park时，先尝试能否直接拿到“许可”，即_counter>0时，如果成功，则把_counter设置为0，并返回。 - 如果不成功，则把线程的状态设置成
_thread_in_vm并且_thread_blocked。_thread_in_vm表示线程当前在JVM中执行，_thread_blocked表示线程当前阻塞了。 - 拿到
mutex之后，再次检查_counter是不是>0，如果是，则把_counter设置为0，unlock mutex并返回。 - 如果
_counter还是不大于0，则判断等待的时间是否等于0，然后调用相应的pthread_cond_wait系列函数进行等待，如果等待返回（即有人进行unpark，则pthread_cond_signal来通知），则把_counter设置为0，unlock mutex并返回。
所以本质上来讲，LockSupport.park是通过pthread库的条件变量pthread_cond_t来实现的。下面我们就来看看pthread_cond_t是怎么实现的。
二、GLIBC层
pthread_cond_t典型的用法如下：
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; /*初始化互斥锁*/
pthread_cond_t cond = PTHREAD_COND_INITIALIZER; //初始化条件变量
void *thread1(void *);
void *thread2(void *);
int i=1;
int main(void)
{
pthread_t t_a;
pthread_t t_b;
pthread_create(&t_a,NULL,thread1,(void *)NULL);/*创建进程t_a*/
pthread_create(&t_b,NULL,thread2,(void *)NULL); /*创建进程t_b*/
pthread_join(t_b, NULL);/*等待进程t_b结束*/
pthread_mutex_destroy(&mutex);
pthread_cond_destroy(&cond);
exit(0);
}
void *thread1(void *junk)
{
for(i=1;i<=9;i++)
{
pthread_mutex_lock(&mutex);//
if(i%3==0)
pthread_cond_signal(&cond);/*条件改变，发送信号，通知t_b进程*/
else
printf("thead1:%d/n",i);
pthread_mutex_unlock(&mutex);//*解锁互斥量*/
printf("Up Unlock Mutex/n");
sleep(1);
}
}
void *thread2(void *junk)
{
while(i<9)
{
pthread_mutex_lock(&mutex);
if(i%3!=0)
pthread_cond_wait(&cond,&mutex);/*等待*/
printf("thread2:%d/n",i);
pthread_mutex_unlock(&mutex);
printf("Down Ulock Mutex/n");
sleep(1);
}
}
重点就是：无论是pthread_cond_wait还是pthread_cond_signal都必须得先pthread_mutex_lock。
如果没有这个保护，可能会产生race condition，漏掉信号。pthread_cond_wait()函数一进入wait状态就会自动release mutex。
当其他线程通过pthread_cond_signal或pthread_cond_broadcast把该线程唤醒，使pthread_cond_wait()返回时，该线程又自动获得该mutex。
整个过程如下图所示：
3.1 pthread_mutex_lock
例如，在Linux中，使用了称为Futex(快速用户空间互斥锁的简称)的系统。
在此系统中，对用户空间中的互斥变量执行原子增量和测试操作。
如果操作结果表明锁上没有争用，则对pthread_mutex_lock的调用将返回，而无需将上下文切换到内核中，因此获取互斥量的操作可以非常快。
仅当检测到争用时，系统调用(称为futex)才会发生，并且上下文切换到内核中，这会使调用进程进入睡眠状态，直到释放互斥锁为止。
还有很多更多的细节，尤其是对于可靠和/或优先级继承互斥，但这就是它的本质。
nptl/pthread_mutex_lock.c
int
PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
{
/* See concurrency notes regarding mutex type which is loaded from __kind
in struct __pthread_mutex_s in sysdeps/nptl/bits/thread-shared-types.h. */
unsigned int type = PTHREAD_MUTEX_TYPE_ELISION (mutex);
LIBC_PROBE (mutex_entry, 1, mutex);
if (__builtin_expect (type & ~(PTHREAD_MUTEX_KIND_MASK_NP
| PTHREAD_MUTEX_ELISION_FLAGS_NP), 0))
return __pthread_mutex_lock_full (mutex);
if (__glibc_likely (type == PTHREAD_MUTEX_TIMED_NP))
{
FORCE_ELISION (mutex, goto elision);
simple:
/* Normal mutex. */
LLL_MUTEX_LOCK_OPTIMIZED (mutex);
assert (mutex->__data.__owner == 0);
}
#if ENABLE_ELISION_SUPPORT
else if (__glibc_likely (type == PTHREAD_MUTEX_TIMED_ELISION_NP))
{
elision: __attribute__((unused))
/* This case can never happen on a system without elision,
as the mutex type initialization functions will not
allow to set the elision flags. */
/* Don't record owner or users for elision case. This is a
tail call. */
return LLL_MUTEX_LOCK_ELISION (mutex);
}
#endif
else if (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex)
== PTHREAD_MUTEX_RECURSIVE_NP, 1))
{
/* Recursive mutex. */
pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
/* Check whether we already hold the mutex. */
if (mutex->__data.__owner == id)
{
/* Just bump the counter. */
if (__glibc_unlikely (mutex->__data.__count + 1 == 0))
/* Overflow of the counter. */
return EAGAIN;
++mutex->__data.__count;
return 0;
}
/* We have to get the mutex. */
LLL_MUTEX_LOCK_OPTIMIZED (mutex);
assert (mutex->__data.__owner == 0);
mutex->__data.__count = 1;
}
else if (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex)
== PTHREAD_MUTEX_ADAPTIVE_NP, 1))
{
if (LLL_MUTEX_TRYLOCK (mutex) != 0)
{
int cnt = 0;
int max_cnt = MIN (max_adaptive_count (),
mutex->__data.__spins * 2 + 10);
do
{
if (cnt++ >= max_cnt)
{
LLL_MUTEX_LOCK (mutex);
break;
}
atomic_spin_nop ();
}
while (LLL_MUTEX_TRYLOCK (mutex) != 0);
mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
}
assert (mutex->__data.__owner == 0);
}
else
{
pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
assert (PTHREAD_MUTEX_TYPE (mutex) == PTHREAD_MUTEX_ERRORCHECK_NP);
/* Check whether we already hold the mutex. */
if (__glibc_unlikely (mutex->__data.__owner == id))
return EDEADLK;
goto simple;
}
pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
/* Record the ownership. */
mutex->__data.__owner = id;
#ifndef NO_INCR
++mutex->__data.__nusers;
#endif
LIBC_PROBE (mutex_acquired, 1, mutex);
return 0;
}
pthread_mutex_t的定义如下：
typedef union
{
struct __pthread_mutex_s
{
int __lock;
unsigned int __count;
int __owner;
unsigned int __nusers;
int __kind;
int __spins;
__pthread_list_t __list;
} __data;
//...
} pthread_mutex_t;
其中__kind字段是指锁的类型，取值如下：
/* Mutex types. */
enum
{
PTHREAD_MUTEX_TIMED_NP,
PTHREAD_MUTEX_RECURSIVE_NP,
PTHREAD_MUTEX_ERRORCHECK_NP,
PTHREAD_MUTEX_ADAPTIVE_NP
#if defined __USE_UNIX98 || defined __USE_XOPEN2K8
,
PTHREAD_MUTEX_NORMAL = PTHREAD_MUTEX_TIMED_NP,
PTHREAD_MUTEX_RECURSIVE = PTHREAD_MUTEX_RECURSIVE_NP,
PTHREAD_MUTEX_ERRORCHECK = PTHREAD_MUTEX_ERRORCHECK_NP,
PTHREAD_MUTEX_DEFAULT = PTHREAD_MUTEX_NORMAL
#endif
#ifdef __USE_GNU
/* For compatibility. */
, PTHREAD_MUTEX_FAST_NP = PTHREAD_MUTEX_TIMED_NP
#endif
};
其中：
- PTHREAD_MUTEX_TIMED_NP，这是缺省值，也就是普通锁。
- PTHREAD_MUTEX_RECURSIVE_NP，可重入锁，允许同一个线程对同一个锁成功获得多次，并通过多次
unlock解锁。 - PTHREAD_MUTEX_ERRORCHECK_NP，检错锁，如果同一个线程重复请求同一个锁，则返回
EDEADLK，否则与PTHREAD_MUTEX_TIMED_NP类型相同。 - PTHREAD_MUTEX_ADAPTIVE_NP，自适应锁，自旋锁与普通锁的混合。
mutex默认用的是PTHREAD_MUTEX_TIMED_NP，所以会走到LLL_MUTEX_LOCK_OPTIMIZED，这是个宏：
# define LLL_MUTEX_LOCK_OPTIMIZED(mutex) lll_mutex_lock_optimized (mutex)
lll_mutex_lock_optimized (pthread_mutex_t *mutex)
{
/* The single-threaded optimization is only valid for private
mutexes. For process-shared mutexes, the mutex could be in a
shared mapping, so synchronization with another process is needed
even without any threads. If the lock is already marked as
acquired, POSIX requires that pthread_mutex_lock deadlocks for
normal mutexes, so skip the optimization in that case as
well. */
int private = PTHREAD_MUTEX_PSHARED (mutex);
if (private == LLL_PRIVATE && SINGLE_THREAD_P && mutex->__data.__lock == 0)
mutex->__data.__lock = 1;
else
lll_lock (mutex->__data.__lock, private);
}
由于不是LLL_PRIVATE，所以走lll_lock，lll_lock也是个宏：
#define lll_lock(futex, private)
__lll_lock (&(futex), private)
注意这里出现了futex，本文的后续主要就是围绕它展开的。
#define __lll_lock(futex, private)
((void)
({
int *__futex = (futex);
if (__glibc_unlikely
(atomic_compare_and_exchange_bool_acq (__futex, 1, 0)))
{
if (__builtin_constant_p (private) && (private) == LLL_PRIVATE)
__lll_lock_wait_private (__futex);
else
__lll_lock_wait (__futex, private);
}
}))
其中，atomic_compare_and_exchange_bool_acq是尝试通过原子操作尝试将__futex（就是mutex->__data.__lock）从0变为1，如果成功就直接返回了，如果失败，则调用__lll_lock_wait，代码如下：
void
__lll_lock_wait(int*futex, int private)
{
if (atomic_load_relaxed (futex) == 2)
goto futex;
while (atomic_exchange_acquire (futex, 2) != 0)
{
futex:
LIBC_PROBE (lll_lock_wait, 1, futex);
futex_wait ((unsigned int *) futex, 2, private); /* Wait if *futex == 2. */
}
}
在这里先要说明一下，pthread将futex的锁状态定义为3种：
- 0，代表当前锁空闲无锁，可以进行快速上锁，不需要进内核。
- 1，代表有线程持有当前锁，如果这时有其它线程需要上锁，就必须标记futex为“锁竞争”，然后通过futex系统调用进内核把当前线程挂起。
- 2，代表锁竞争，有其它线程将要或正在内核的futex系统中排队等待锁。
所以上锁失败进入到__lll_lock_wait这里后，先判断futex是不是等于2，如果是则说明大家都在排队，你也排着吧（直跳转到futex_wait）。如果不等于2，那说明你是第一个来竞争的人，把futex设置成2，告诉后面来的人要排队，然后自己以身作则先排队。
futex_wait实质上就是调用futex系统调用。在第四节，我们就来仔细分析这个系统调用。
3.2 pthread_cond_wait
本质也是走到futex系统调用，限于篇幅就不展开了。
三、内核层
为什么要有futex，它解决什么问题？何时加入内核的？
简单来讲，futex的解决思路是：在无竞争的情况下操作完全在user space进行，不需要系统调用，仅在发生竞争的时候进入内核去完成相应的处理(wait或者wake up)。所以说，futex是一种user mode和kernel mode混合的同步机制，需要两种模式合作才能完成，futex变量位于user space，而不是内核对象，futex的代码也分为user mode和kernel mode两部分，无竞争的情况下在user mode，发生竞争时则通过sys_futex系统调用进入kernel mode进行处理。
用户态的部分已经在前面讲解了，本节重点讲解futex在内核部分的实现。
futex设计了三个基本数据结构：futex_hash_bucket，futex_key，futex_q。
struct futex_hash_bucket {
atomic_t waiters;
spinlock_t lock;
struct plist_head chain;
} ____cacheline_aligned_in_smp;
struct futex_q {
struct plist_node list;
struct task_struct *task;
spinlock_t *lock_ptr;
union futex_key key; //唯一标识uaddr的key值
struct futex_pi_state *pi_state;
struct rt_mutex_waiter *rt_waiter;
union futex_key *requeue_pi_key;
u32 bitset;
};
union futex_key {
struct {
unsigned long pgoff;
struct inode *inode;
int offset;
} shared;
struct {
unsigned long address;
struct mm_struct *mm;
int offset;
} private;
struct {
unsigned long word;
void *ptr;
int offset;
} both;
};
其实还有个struct __futex_data，如下所示，这个
static struct {
struct futex_hash_bucket *queues;
unsigned long hashsize;
} __futex_data __read_mostly __aligned(2*sizeof(long));
#define futex_queues (__futex_data.queues)
#define futex_hashsize (__futex_data.hashsize)
在futex初始化的时候（futex_init），会确定hashsize，比如24核cpu时，hashsize = 8192。
然后根据这个hashsize调用alloc_large_system_hash分配数组空间，并初始化数组元素里的相关字段，比如plist_head,lock。
static int __init futex_init(void)
{
unsigned int futex_shift;
unsigned long i;
#if CONFIG_BASE_SMALL
futex_hashsize = 16;
#else
futex_hashsize = roundup_pow_of_two(256 * num_possible_cpus());
#endif
futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
futex_hashsize, 0,
futex_hashsize < 256 ? HASH_SMALL : 0,
&futex_shift, NULL,
futex_hashsize, futex_hashsize);
futex_hashsize = 1UL << futex_shift;
futex_detect_cmpxchg();
for (i = 0; i < futex_hashsize; i++) {
atomic_set(&futex_queues[i].waiters, 0);
plist_head_init(&futex_queues[i].chain);
spin_lock_init(&futex_queues[i].lock);
}
return 0;
}
这些数据结构之间的关系如下所示：
脑子里有了数据结构，流程就容易理解了。futex_wait的总体流程如下：
static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
ktime_t *abs_time, u32 bitset)
{
struct hrtimer_sleeper timeout, *to = NULL;
struct restart_block *restart;
struct futex_hash_bucket *hb;
struct futex_q q = futex_q_init;
int ret;
if (!bitset)
return -EINVAL;
q.bitset = bitset;
if (abs_time) {
to = &timeout;
hrtimer_init_on_stack(&to->timer, (flags & FLAGS_CLOCKRT) ?
CLOCK_REALTIME : CLOCK_MONOTONIC,
HRTIMER_MODE_ABS);
hrtimer_init_sleeper(to, current);
hrtimer_set_expires_range_ns(&to->timer, *abs_time,
current->timer_slack_ns);
}
retry:
/*
* Prepare to wait on uaddr. On success, holds hb lock and increments
* q.key refs.
*/
ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
if (ret)
goto out;
/* queue_me and wait for wakeup, timeout, or a signal. */
futex_wait_queue_me(hb, &q, to);
/* If we were woken (and unqueued), we succeeded, whatever. */
ret = 0;
/* unqueue_me() drops q.key ref */
if (!unqueue_me(&q))
goto out;
ret = -ETIMEDOUT;
if (to && !to->task)
goto out;
/*
* We expect signal_pending(current), but we might be the
* victim of a spurious wakeup as well.
*/
if (!signal_pending(current))
goto retry;
ret = -ERESTARTSYS;
if (!abs_time)
goto out;
restart = ¤t->restart_block;
restart->fn = futex_wait_restart;
restart->futex.uaddr = uaddr;
restart->futex.val = val;
restart->futex.time = *abs_time;
restart->futex.bitset = bitset;
restart->futex.flags = flags | FLAGS_HAS_TIMEOUT;
ret = -ERESTART_RESTARTBLOCK;
out:
if (to) {
hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
}
return ret;
}
函数futex_wait_setup主要做两件事，
一是对uaddr进行hash，找到futex_hash_bucket并获取它上面的自旋锁，
二是判断*uaddr是否为预期值。如果不相等则会立即返回，由用户态继续trylock。
/*
* futex_wait_setup() - Prepare to wait on a futex
* @uaddr: the futex userspace address
* @val: the expected value
* @flags: futex flags (FLAGS_SHARED, etc.)
* @q: the associated futex_q
* @hb: storage for hash_bucket pointer to be returned to caller
*
* Setup the futex_q and locate the hash_bucket. Get the futex value and
* compare it with the expected value. Handle atomic faults internally.
* Return with the hb lock held and a q.key reference on success, and unlocked
* with no q.key reference on failure.
*
* Return:
* - 0 - uaddr contains val and hb has been locked;
* - <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlocked
*/
static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
struct futex_q *q, struct futex_hash_bucket **hb)
{
u32 uval;
int ret;
retry:
//初始化futex_q， 把uaddr设置到futex_key的字段中，将来futex_wake时也是通过这个key来查找futex。
ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key, VERIFY_READ);
if (unlikely(ret != 0))
return ret;
retry_private:
//根据key计算hash，然后在数组里找到对应的futex_hash_bucket
*hb = queue_lock(q);
//原子地将uaddr的值读到uval中
ret = get_futex_value_locked(&uval, uaddr);
if (ret) {
queue_unlock(*hb);
ret = get_user(uval, uaddr);
if (ret)
goto out;
if (!(flags & FLAGS_SHARED))
goto retry_private;
put_futex_key(&q->key);
goto retry;
}
//如果当前uaddr指向的值不等于val，即说明其他进程修改了
//uaddr指向的值，等待条件不再成立，不用阻塞直接返回。
if (uval != val) {
queue_unlock(*hb);
ret = -EWOULDBLOCK;
}
out:
if (ret)
put_futex_key(&q->key);
return ret;
}
然后调用futex_wait_queue_me把当前进程挂起：
/**
* futex_wait_queue_me() - queue_me() and wait for wakeup, timeout, or signal
* @hb: the futex hash bucket, must be locked by the caller
* @q: the futex_q to queue up on
* @timeout: the prepared hrtimer_sleeper, or null for no timeout
*/
static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
struct hrtimer_sleeper *timeout)
{
/*
* The task state is guaranteed to be set before another task can
* wake it. set_current_state() is implemented using smp_store_mb() and
* queue_me() calls spin_unlock() upon completion, both serializing
* access to the hash list and forcing another memory barrier.
*/
set_current_state(TASK_INTERRUPTIBLE);
queue_me(q, hb);
/* Arm the timer */
if (timeout)
hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
/*
* If we have been removed from the hash list, then another task
* has tried to wake us, and we can skip the call to schedule().
*/
if (likely(!plist_node_empty(&q->list))) {
/*
* If the timer has already expired, current will already be
* flagged for rescheduling. Only call schedule if there
* is no timeout, or if it has yet to expire.
*/
if (!timeout || timeout->task)
freezable_schedule();
}
__set_current_state(TASK_RUNNING);
}
futex_wait_queue_me主要做几件事：
- 将当前进程插入到等待队列，就是把
futex_q挂到futex_hash_bucket上 - 启动定时任务
- 主动触发内核进程调度
