记录linux tty的一次软锁排查2
在复现tty的死锁问题的时候，文洋兄使用了如下的方式：
#include <fcntl.h> #include <unistd.h> #include <stdio.h> #define TIOCVHANGUP 0x5437 int main(int argc,char* argv[]) { int fd; if(argc < 2) { printf("error,you should input tty as a parameter\r\n"); return 1; } fd = open(argv[1], O_WRONLY | O_NOCTTY);
if(fd<0)
{
return 1;
}
write(fd, "test tty\n ", 20); ioctl(fd, TIOCVHANGUP, 0); //sleep(1); close(fd); return 0; }
编译成gcc -g -o main.o main.c ,然后使用脚本呼叫：
#!/bin/bash while [ 1 ] do ./main.o /dev/tty4 done
之所以使用脚本而不是在c中while处理，是因为在进程exit的时候，会有些tty的处理,我们希望尽可能地覆盖测试，所以甚至都没有加sleep来延时。
结果复现出来下面的软锁故障，堆栈如下：
[517571.855382] INFO: task systemd:1 blocked for more than 120 seconds. [517571.856127] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [517571.856846] systemd D ffff881fffc347c0 0 1 0 0x00000000 [517571.856852] ffff881fd35c7b50 0000000000000086 ffff881fd35c7fd8 ffff881fd35c7fd8 [517571.856859] ffff881fd35c7fd8 00000000000147c0 ffff881fd313c500 ffff883f5ee2ac80 [517571.856863] ffff883f5ee2ac84 ffff883fd1630000 00000000ffffffff ffff883f5ee2ac88 [517571.856867] Call Trace: [517571.856880] [<ffffffff8163f959>] schedule_preempt_disabled+0x29/0x70 [517571.856883] [<ffffffff8163d415>] __mutex_lock_slowpath+0xc5/0x1c0 [517571.856888] [<ffffffff8163c87f>] mutex_lock+0x1f/0x2f [517571.856890] [<ffffffff81640df8>] tty_lock_nested.isra.0+0x38/0x90 [517571.856892] [<ffffffff81640e5e>] tty_lock+0xe/0x10 [517571.856899] [<ffffffff813b204c>] tty_open+0xcc/0x620 [517571.856906] [<ffffffff811e5721>] chrdev_open+0xa1/0x1e0 [517571.856912] [<ffffffff811de657>] do_dentry_open+0x1a7/0x2e0 [517571.856916] [<ffffffff811e5680>] ? cdev_put+0x30/0x30 [517571.856918] [<ffffffff811de889>] vfs_open+0x39/0x70 [517571.856922] [<ffffffff811ede7d>] do_last+0x1ed/0x1270 [517571.856925] [<ffffffff811f0be2>] path_openat+0xc2/0x490 [517571.856930] [<ffffffff810afb68>] ? __wake_up_common+0x58/0x90 [517571.856935] [<ffffffff811f23ab>] do_filp_open+0x4b/0xb0 [517571.856941] [<ffffffff811fef47>] ? __alloc_fd+0xa7/0x130 [517571.856945] [<ffffffff811dfd53>] do_sys_open+0xf3/0x1f0 [517571.856949] [<ffffffff811dfe6e>] SyS_open+0x1e/0x20 [517571.856955] [<ffffffff81649909>] system_call_fastpath+0x16/0x1b
从堆栈看，显然又是在等锁超时了。反汇编找到这把锁是关键。
void __lockfunc tty_lock(struct tty_struct *tty) { return tty_lock_nested(tty, TTY_MUTEX_NORMAL); }
static void __lockfunc tty_lock_nested(struct tty_struct *tty, unsigned int subclass) { if (tty->magic != TTY_MAGIC) { pr_err("L Bad %p\n", tty); WARN_ON(1); return; } tty_kref_get(tty); mutex_lock_nested(&tty->legacy_mutex, subclass);--------------传入锁的指针 }
由于CONFIG_DEBUG_LOCK_ALLOC并没有配置，所以mutex_lock_nested就是mutex_lock。和堆栈是匹配的。
# define mutex_lock_nested(lock, subclass) mutex_lock(lock)
crash> dis -l tty_lock_nested /usr/src/debug/kernel-3.10.0-327.22.2.el7/linux-3.10.0-327.22.2.el7.x86_64/drivers/tty/tty_mutex.c: 18 0xffffffff81640dc0 <tty_lock_nested>: nopl 0x0(%rax,%rax,1) [FTRACE NOP] 0xffffffff81640dc5 <tty_lock_nested+5>: push %rbp 0xffffffff81640dc6 <tty_lock_nested+6>: mov %rsp,%rbp 0xffffffff81640dc9 <tty_lock_nested+9>: push %rbx /usr/src/debug/kernel-3.10.0-327.22.2.el7/linux-3.10.0-327.22.2.el7.x86_64/drivers/tty/tty_mutex.c: 21 0xffffffff81640dca <tty_lock_nested+10>: cmpl $0x5401,(%rdi) /usr/src/debug/kernel-3.10.0-327.22.2.el7/linux-3.10.0-327.22.2.el7.x86_64/drivers/tty/tty_mutex.c: 18 0xffffffff81640dd0 <tty_lock_nested+16>: mov %rdi,%rbx /usr/src/debug/kernel-3.10.0-327.22.2.el7/linux-3.10.0-327.22.2.el7.x86_64/drivers/tty/tty_mutex.c: 21 0xffffffff81640dd3 <tty_lock_nested+19>: jne 0xffffffff81640dfb <tty_lock_nested+59> /usr/src/debug/kernel-3.10.0-327.22.2.el7/linux-3.10.0-327.22.2.el7.x86_64/include/linux/tty.h: 388 0xffffffff81640dd5 <tty_lock_nested+21>: test %rdi,%rdi 0xffffffff81640dd8 <tty_lock_nested+24>: je 0xffffffff81640dec <tty_lock_nested+44> /usr/src/debug/kernel-3.10.0-327.22.2.el7/linux-3.10.0-327.22.2.el7.x86_64/arch/x86/include/asm/atomic.h: 176 0xffffffff81640dda <tty_lock_nested+26>: mov $0x1,%eax 0xffffffff81640ddf <tty_lock_nested+31>: lock xadd %eax,0x4(%rdi) 0xffffffff81640de4 <tty_lock_nested+36>: add $0x1,%eax /usr/src/debug/kernel-3.10.0-327.22.2.el7/linux-3.10.0-327.22.2.el7.x86_64/include/linux/kref.h: 47 0xffffffff81640de7 <tty_lock_nested+39>: cmp $0x1,%eax 0xffffffff81640dea <tty_lock_nested+42>: jle 0xffffffff81640e1f <tty_lock_nested+95> /usr/src/debug/kernel-3.10.0-327.22.2.el7/linux-3.10.0-327.22.2.el7.x86_64/drivers/tty/tty_mutex.c: 27 0xffffffff81640dec <tty_lock_nested+44>: lea 0x80(%rbx),%rdi------------------传入的参数是一把锁的地址，即&tty->legacy_mutex，rbx就是tty的指针了。
0xffffffff81640df3 <tty_lock_nested+51>: callq 0xffffffff8163c860 <mutex_lock>--------------------调用mutex_lock
crash> dis -l mutex_lock /usr/src/debug/kernel-3.10.0-327.22.2.el7/linux-3.10.0-327.22.2.el7.x86_64/kernel/mutex.c: 103 0xffffffff8163c860 <mutex_lock>: nopl 0x0(%rax,%rax,1) [FTRACE NOP] 0xffffffff8163c865 <mutex_lock+5>: push %rbp 0xffffffff8163c866 <mutex_lock+6>: mov %rsp,%rbp 0xffffffff8163c869 <mutex_lock+9>: push %rbx--------------------------------------------------rbx压栈,所以rbp后面就是rbx的值
所以我们能够通过堆栈分析出tty的指针来，rbx的压栈的位置是在rbp之后。
ffff881fd35c7bc0: ffff881fd35c7bd8 ffffffff8163c87f #3 [ffff881fd35c7bc8] mutex_lock at ffffffff8163c87f ffff881fd35c7bd0: ffff883f5ee2ac00 ffff881fd35c7bf0 -----------------------ffff883f5ee2ac00就是rbx的值，也就是tty指针 ffff881fd35c7be0: ffffffff81640df8 #4 [ffff881fd35c7be0] tty_lock_nested at ffffffff81640df8 ffff881fd35c7be8: ffff88211f6a3200 ffff881fd35c7c00 ffff881fd35c7bf8: ffffffff81640e5e
现在，需要找到持有这把锁的owner是谁。
crash> struct tty_struct.legacy_mutex ffff883f5ee2ac00 legacy_mutex = { count = { counter = -1 }, wait_lock = { { rlock = { raw_lock = { { head_tail = 524296, tickets = { head = 8, tail = 8 } } } } } }, wait_list = { next = 0xffff881fd35c7b70, prev = 0xffff881fd35c7b70 }, owner = 0xffff880190f5c500, -----------------持有锁
查看对应的task：
crash> task 0xffff880190f5c500 PID: 5628 TASK: ffff880190f5c500 CPU: 47 COMMAND: "main.o"------------就是我们编译的测试命令
确认下是不是我们的tty4.
crash> struct tty_strt.name ffff883f5ee2ac00 name = "tty4\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"
确定无误后，看看进程打开的文件列表：
crash> files 5628 PID: 5628 TASK: ffff880190f5c500 CPU: 47 COMMAND: "main.o" ROOT: / CWD: /home/caq FD FILE DENTRY INODE TYPE PATH 0 ffff881f0e31a600 ffff880dd37f8000 ffff8801713fcea0 CHR /dev/pts/45 1 ffff881f0e31a600 ffff880dd37f8000 ffff8801713fcea0 CHR /dev/pts/45 2 ffff881f0e31a600 ffff880dd37f8000 ffff8801713fcea0 CHR /dev/pts/45 3 ffff881a00324400 ffff883fd1010fc0 ffff883fd0b73820 CHR /dev/tty4
查看对应的tty的属性：
crash> struct file.private_data ffff881a00324400 private_data = 0xffff883f6101e840 crash> struct tty_file_private.tty 0xffff883f6101e840 tty = 0xffff883f5ee2ac00 crash> struct tty_struct.disc_data 0xffff883f5ee2ac00----------------这个 0xffff883f5ee2ac00 也就是在前面反汇编找到的tty指针
disc_data = 0xffff883f9a1d8c00
crash> struct n_tty_data.icanon 0xffff883f9a1d8c00 icanon = 1 '\001'
当然也可以使用tty来直接查看。
最后殊途同归，还是同一个问题，属性导致的。
我们继续来看到底有多少进程被阻塞了：
# grep mutex_lock -A 5 -B 5 caq_all_bt.txt |grep tty_open |wc -l
90
# grep mutex_lock -A 5 -B 5 caq_all_bt.txt |grep tty_open #6 [ffff881fd35c7c08] tty_open at ffffffff813b204c-----------------只有1号进程阻塞在这 #3 [ffff8820c222bc08] tty_open at ffffffff813b1ff7-----------------其余全部阻塞在这 #3 [ffff882aaa9b3c08] tty_open at ffffffff813b1ff7 #3 [ffff883f20ca7c08] tty_open at ffffffff813b1ff7 #3 [ffff882098d2bc08] tty_open at ffffffff813b1ff7 #3 [ffff88147ff87c08] tty_open at ffffffff813b1ff7 #3 [ffff8820ff4cbc08] tty_open at ffffffff813b1ff7 #3 [ffff88106e5c7c08] tty_open at ffffffff813b1ff7 #3 [ffff880192813c08] tty_open at ffffffff813b1ff7 #3 [ffff880164ccbc08] tty_open at ffffffff813b1ff7 #3 [ffff882093c13c08] tty_open at ffffffff813b1ff7 #3 [ffff8814221b7c08] tty_open at ffffffff813b1ff7 #3 [ffff883f3c74fc08] tty_open at ffffffff813b1ff7 #3 [ffff88136e433c08] tty_open at ffffffff813b1ff7 #3 [ffff882141f37c08] tty_open at ffffffff813b1ff7 #3 [ffff8820db4ebc08] tty_open at ffffffff813b1ff7 #3 [ffff88149471fc08] tty_open at ffffffff813b1ff7 #3 [ffff8801a4417c08] tty_open at ffffffff813b1ff7 #3 [ffff883f0acd3c08] tty_open at ffffffff813b1ff7 #3 [ffff883ebce9fc08] tty_open at ffffffff813b1ff7 #3 [ffff88208bfd3c08] tty_open at ffffffff813b1ff7 #3 [ffff882087d0bc08] tty_open at ffffffff813b1ff7 #3 [ffff8820d556bc08] tty_open at ffffffff813b1ff7 #3 [ffff8820c235bc08] tty_open at ffffffff813b1ff7 #3 [ffff8820e7ce3c08] tty_open at ffffffff813b1ff7 #3 [ffff88210c25fc08] tty_open at ffffffff813b1ff7 #3 [ffff8820ebe2fc08] tty_open at ffffffff813b1ff7 #3 [ffff8820e82c7c08] tty_open at ffffffff813b1ff7 #3 [ffff88212af2fc08] tty_open at ffffffff813b1ff7 #3 [ffff881ad4ef7c08] tty_open at ffffffff813b1ff7 #3 [ffff883f1a8afc08] tty_open at ffffffff813b1ff7 #3 [ffff88146efb3c08] tty_open at ffffffff813b1ff7 #3 [ffff8801c557fc08] tty_open at ffffffff813b1ff7 #3 [ffff88044e66fc08] tty_open at ffffffff813b1ff7 #3 [ffff8801664dbc08] tty_open at ffffffff813b1ff7 #3 [ffff8801a1fefc08] tty_open at ffffffff813b1ff7 #3 [ffff8801850c7c08] tty_open at ffffffff813b1ff7 #3 [ffff8801c6563c08] tty_open at ffffffff813b1ff7 #3 [ffff8801751dfc08] tty_open at ffffffff813b1ff7 #3 [ffff8801272fbc08] tty_open at ffffffff813b1ff7 #3 [ffff880173073c08] tty_open at ffffffff813b1ff7 #3 [ffff880179ccbc08] tty_open at ffffffff813b1ff7 #3 [ffff8813895f7c08] tty_open at ffffffff813b1ff7 #3 [ffff88152025fc08] tty_open at ffffffff813b1ff7 #3 [ffff88019e403c08] tty_open at ffffffff813b1ff7 #3 [ffff8801504f3c08] tty_open at ffffffff813b1ff7 #3 [ffff88017841fc08] tty_open at ffffffff813b1ff7 #3 [ffff88018e80fc08] tty_open at ffffffff813b1ff7 #3 [ffff881345b57c08] tty_open at ffffffff813b1ff7 #3 [ffff881f2c0ffc08] tty_open at ffffffff813b1ff7 #3 [ffff88049b78bc08] tty_open at ffffffff813b1ff7 #3 [ffff8801aff13c08] tty_open at ffffffff813b1ff7 #3 [ffff880186f77c08] tty_open at ffffffff813b1ff7 #3 [ffff8814fd963c08] tty_open at ffffffff813b1ff7 #3 [ffff8803d37dbc08] tty_open at ffffffff813b1ff7 #3 [ffff8801cacfbc08] tty_open at ffffffff813b1ff7 #3 [ffff8801d6937c08] tty_open at ffffffff813b1ff7 #3 [ffff8805689d3c08] tty_open at ffffffff813b1ff7 #3 [ffff883f8b9d7c08] tty_open at ffffffff813b1ff7 #3 [ffff883f7d873c08] tty_open at ffffffff813b1ff7 #3 [ffff8801fd47bc08] tty_open at ffffffff813b1ff7 #3 [ffff881387ecfc08] tty_open at ffffffff813b1ff7 #3 [ffff88145225fc08] tty_open at ffffffff813b1ff7 #3 [ffff88055235bc08] tty_open at ffffffff813b1ff7 #3 [ffff8803d2297c08] tty_open at ffffffff813b1ff7 #3 [ffff881432223c08] tty_open at ffffffff813b1ff7 #3 [ffff880d100cbc08] tty_open at ffffffff813b1ff7 #3 [ffff88018e9e3c08] tty_open at ffffffff813b1ff7 #3 [ffff8813879d7c08] tty_open at ffffffff813b1ff7 #3 [ffff88021a327c08] tty_open at ffffffff813b1ff7 #3 [ffff88021747bc08] tty_open at ffffffff813b1ff7 #3 [ffff88016bb43c08] tty_open at ffffffff813b1ff7 #3 [ffff880152223c08] tty_open at ffffffff813b1ff7 #3 [ffff8801acbcbc08] tty_open at ffffffff813b1ff7 #3 [ffff88018a2dfc08] tty_open at ffffffff813b1ff7 #3 [ffff88018821bc08] tty_open at ffffffff813b1ff7 #3 [ffff883ea5b9bc08] tty_open at ffffffff813b1ff7 #3 [ffff880242e8fc08] tty_open at ffffffff813b1ff7 #3 [ffff88136ce7fc08] tty_open at ffffffff813b1ff7 #3 [ffff880186217c08] tty_open at ffffffff813b1ff7 #3 [ffff8801685b3c08] tty_open at ffffffff813b1ff7 #3 [ffff883edb1bbc08] tty_open at ffffffff813b1ff7 #3 [ffff883efc4dfc08] tty_open at ffffffff813b1ff7 #3 [ffff8820ecaffc08] tty_open at ffffffff813b1ff7 #3 [ffff883e77557c08] tty_open at ffffffff813b1ff7 #3 [ffff8813dcbdfc08] tty_open at ffffffff813b1ff7 #3 [ffff8801544dfc08] tty_open at ffffffff813b1ff7 #3 [ffff8820d552fc08] tty_open at ffffffff813b1ff7 #3 [ffff8801dab0fc08] tty_open at ffffffff813b1ff7 #3 [ffff883fa1f83c08] tty_open at ffffffff813b1ff7
这90个中，只有一个是#6 [ffff881fd35c7c08] tty_open at ffffffff813b204c，其他都是阻塞在tty_open at ffffffff813b1ff7，根据反汇编的行号，说明89个进程在
mutex_lock(&tty_mutex);阻塞。这是一把大锁。
这89个进程阻塞的原因是1号进程拿到了tty_mutex这把大的互斥锁。
然后1号进程被阻塞在
if (tty) { tty_lock(tty);--------------------1号进程阻塞在这，即阻塞在tty->legacy_mutex 锁。
retval = tty_reopen(tty); if (retval < 0) { tty_unlock(tty); tty = ERR_PTR(retval); }
1号进程阻塞是因为5628进程，来看一下5628进程的堆栈：
#0 [ffff883edb11fbd0] __schedule at ffffffff8163df9b #1 [ffff883edb11fc38] schedule at ffffffff8163e879 #2 [ffff883edb11fc48] schedule_timeout at ffffffff8163c329 #3 [ffff883edb11fcf8] ldsem_down_write at ffffffff8164061a #4 [ffff883edb11fd68] tty_ldisc_lock_pair_timeout at ffffffff81640cd8 #5 [ffff883edb11fd98] tty_ldisc_hangup at ffffffff813b8dc4 #6 [ffff883edb11fdc0] __tty_hangup at ffffffff813b0594 #7 [ffff883edb11fe10] tty_ioctl at ffffffff813b2e55 #8 [ffff883edb11feb8] do_vfs_ioctl at ffffffff811f4465 #9 [ffff883edb11ff30] sys_ioctl at ffffffff811f46e1 #10 [ffff883edb11ff80] system_call_fastpath at ffffffff81649909 RIP: 00007f5438b3f537 RSP: 00007ffef141f478 RFLAGS: 00010206 RAX: 0000000000000010 RBX: ffffffff81649909 RCX: 00007f5438b39c90 RDX: 0000000000000000 RSI: 0000000000005437 RDI: 0000000000000003 RBP: 00007ffef141f4a0 R8: 00007f5438e0ce80 R9: 0000000000000000 R10: 00007ffef141f200 R11: 0000000000000206 R12: 0000000000000000 R13: 0000000000000000 R14: 00007ffef141f580 R15: 0000000000400560 ORIG_RAX: 0000000000000010 CS: 0033 SS: 002b
从下面的__tty_hangup的代码看出，调用tty_ldisc_hangup 前，因为调用了tty_lock(tty);那么确实持有了一把tty->legacy_mutex .
static void __tty_hangup(struct tty_struct *tty, int exit_session) { struct file *cons_filp = NULL; struct file *filp, *f = NULL; struct tty_file_private *priv; int closecount = 0, n; int refs; if (!tty) return; spin_lock(&redirect_lock); if (redirect && file_tty(redirect) == tty) { f = redirect; redirect = NULL; } spin_unlock(&redirect_lock); tty_lock(tty);-----------------------------------------加锁 /* some functions below drop BTM, so we need this bit */ set_bit(TTY_HUPPING, &tty->flags); /* inuse_filps is protected by the single tty lock, this really needs to change if we want to flush the workqueue with the lock held */ check_tty_count(tty, "tty_hangup"); spin_lock(&tty_files_lock); /* This breaks for file handles being sent over AF_UNIX sockets ? */ list_for_each_entry(priv, &tty->tty_files, list) { filp = priv->file; if (filp->f_op->write == redirected_tty_write) cons_filp = filp; if (filp->f_op->write != tty_write) continue; closecount++; __tty_fasync(-1, filp, 0); /* can't block */ filp->f_op = &hung_up_tty_fops; } spin_unlock(&tty_files_lock); refs = tty_signal_session_leader(tty, exit_session); /* Account for the p->signal references we killed */ while (refs--) tty_kref_put(tty); /* * it drops BTM and thus races with reopen * we protect the race by TTY_HUPPING */ tty_ldisc_hangup(tty);-----------------------阻塞，阻塞的原因上面已经描述了。 spin_lock_irq(&tty->ctrl_lock); clear_bit(TTY_THROTTLED, &tty->flags); clear_bit(TTY_PUSH, &tty->flags); clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags); put_pid(tty->session); put_pid(tty->pgrp); tty->session = NULL; tty->pgrp = NULL; tty->ctrl_status = 0; spin_unlock_irq(&tty->ctrl_lock); /* * If one of the devices matches a console pointer, we * cannot just call hangup() because that will cause * tty->count and state->count to go out of sync. * So we just call close() the right number of times. */ if (cons_filp) { if (tty->ops->close) for (n = 0; n < closecount; n++) tty->ops->close(tty, cons_filp); } else if (tty->ops->hangup) (tty->ops->hangup)(tty); /* * We don't want to have driver/ldisc interactions beyond * the ones we did here. The driver layer expects no * calls after ->hangup() from the ldisc side. However we * can't yet guarantee all that. */ set_bit(TTY_HUPPED, &tty->flags); clear_bit(TTY_HUPPING, &tty->flags); tty_unlock(tty);-------------------------------导致没有走到这放锁。 if (f) fput(f); }
本来以为分析已经完成了，结果看了一下tty_ldisc_hangup的代码，又推翻了自己的判断。下面，我们先来看一下tty_ldisc_hangup运行到哪行代码。
crash> dis -l ffffffff813b8dc4 /usr/src/debug/kernel-3.10.0-327.22.2.el7/linux-3.10.0-327.22.2.el7.x86_64/drivers/tty/tty_ldisc.c: 690 0xffffffff813b8dc4 <tty_ldisc_hangup+196>: cmpq $0x0,0x50(%rbx)
690行刚好就是tty_ldisc_lock_pair，也就是tty_ldisc_lock_pair_timeout(tty, tty2, MAX_SCHEDULE_TIMEOUT);
我们看下tty_ldisc_hangup的代码：
void tty_ldisc_hangup(struct tty_struct *tty) { struct tty_ldisc *ld; int reset = tty->driver->flags & TTY_DRIVER_RESET_TERMIOS; int err = 0; tty_ldisc_debug(tty, "closing ldisc: %p\n", tty->ldisc); ld = tty_ldisc_ref(tty); if (ld != NULL) { if (ld->ops->flush_buffer) ld->ops->flush_buffer(tty); tty_driver_flush_buffer(tty); if ((test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) && ld->ops->write_wakeup) ld->ops->write_wakeup(tty); if (ld->ops->hangup) ld->ops->hangup(tty); tty_ldisc_deref(ld); } wake_up_interruptible_poll(&tty->write_wait, POLLOUT); wake_up_interruptible_poll(&tty->read_wait, POLLIN); tty_unlock(tty);------------------------这里明明释放了锁 /* * Shutdown the current line discipline, and reset it to * N_TTY if need be. * * Avoid racing set_ldisc or tty_ldisc_release */ tty_ldisc_lock_pair(tty, tty->link);--------------------690行，也就是tty_ldisc_lock_pair_timeout(tty, tty2, MAX_SCHEDULE_TIMEOUT);跟堆栈一致。 tty_lock(tty);--------------------------重新加上锁 if (tty->ldisc) { /* At this point we have a halted ldisc; we want to close it and reopen a new ldisc. We could defer the reopen to the next open but it means auditing a lot of other paths so this is a FIXME */ if (reset == 0) { if (!tty_ldisc_reinit(tty, tty->termios.c_line)) err = tty_ldisc_open(tty, tty->ldisc); else err = 1; } /* If the re-open fails or we reset then go to N_TTY. The N_TTY open cannot fail */ if (reset || err) { BUG_ON(tty_ldisc_reinit(tty, N_TTY)); WARN_ON(tty_ldisc_open(tty, tty->ldisc)); } } tty_ldisc_enable_pair(tty, tty->link); if (reset) tty_reset_termios(tty); tty_ldisc_debug(tty, "re-opened ldisc: %p\n", tty->ldisc); }
这说明，明明5628进程释放了tty->legacy_mutex啊，为什么1号进程的互斥锁的owner还指向它呢？这个留在下次单独对互斥信号来描述。
我们再次回到那把tty->legacy_mutex锁，
wait_list = { next = 0xffff881fd35c7b70, prev = 0xffff881fd35c7b70 },
list -s mutex_waiter.task 0xffff881fd35c7b70 ffff881fd35c7b70 task = 0xffff883fd1630000 ffff883f5ee2ac88 task = 0xffff880190f5c500
crash> task 0xffff883fd1630000 PID: 1 TASK: ffff883fd1630000 CPU: 1 COMMAND: "systemd"
crash> task 0xffff880190f5c500 PID: 5628 TASK: ffff880190f5c500 CPU: 47 COMMAND: "main.o"
5628怎么可能既是owner，又是waiter呢？这个问题我们放到后面来解释。
crash> struct tty_struct.link ffff883f5ee2ac00 link = 0x0
所以后面的调用链就是：tty_ldisc_lock_pair(tty, tty->link);---->tty_ldisc_lock_pair_timeout(0xffff883f5ee2ac00,0,MAX_SCHEDULE_TIMEOUT)--->tty_ldisc_lock--->ldsem_down_write
5628阻塞在线路规程的锁，也就是tty->ldisc_sem,这个是一把读写锁，在没打开debug的情况下，是没有owner成员的。
crash> struct tty_struct.ldisc_sem ffff883f5ee2ac00 ldisc_sem = { count = -8589934591, wait_lock = { raw_lock = { { head_tail = 655370, tickets = { head = 10, tail = 10 } } } }, wait_readers = 1, read_wait = { next = 0xffff8801846d3df0, prev = 0xffff8801846d3df0 }, write_wait = { next = 0xffff883edb11fd10, prev = 0xffff883edb11fd10 } }
要找到owner，又得人肉遍历堆栈了。和《记录linux tty的一次软锁排查》一样，也是占用了锁，但本来的意愿是占用200ms超时，由于属性被修改，导致了占用无限时间。
那么，很显然，这个测试脚本，可以测试《记录linux tty的一次软锁排查》中的修改是否已经ok。
修改脚本如下：
#!/bin/bash while [ 1 ] do for i in {1..64} do ./main.o /dev/tty$i done done
之前，在未修改fd为noblock的时候，是必现，改完之后，暴力测试一天都正常。
下面，针对前面所说的为什么wait里面看到的task和owner是同一个这个问题，再进行下解释。
tty_init_dev初始化一个tty的时候，调用initialize_tty_struct------>mutex_init(&tty->legacy_mutex);---->__mutex_init,代码如下
__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key) { atomic_set(&lock->count, 1); spin_lock_init(&lock->wait_lock); INIT_LIST_HEAD(&lock->wait_list); mutex_clear_owner(lock); #ifdef CONFIG_MUTEX_SPIN_ON_OWNER lock->osq = NULL; #endif debug_mutex_init(lock, name, key); }
此时，lock的wait_list只包含一个头结点，也就是&lock->wait_list，也就是0xffff881fd35c7b70
此时如果用list -s mutex_waiter.task 0xffff881fd35c7b70 去查看，那么对应的task是NULL。
我们来看mutex_waiter的结构：
struct mutex_waiter { struct list_head list; struct task_struct *task; #ifdef CONFIG_DEBUG_MUTEXES void *magic; #endif };
本来lock->wait_list把mutex_waiter 串起来，而在struct mutex结构中，owner成员刚好就位于struct list_head wait_list的后面，所以当owner获取锁之后，设置owner指针，刚好
就是和mutex_waiter 中设置task一样，所以这次看到的互斥锁的list中，使用list方法查看，会出现owner和wait指向同一个task的现象。
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