文章目录
- 1.死锁eg
1.死锁eg
- 1：代码
下面是一个简单的能够发生死锁的代码：
#include <unistd.h>
#include <pthread.h>
#include <string.h>
typedef struct
{
pthread_mutex_t mutex1;
pthread_mutex_t mutex2;
int sequence1;
int sequence2;
}Counter;
void* thread1(void* arg)
{
Counter *cc = (Counter *)arg;
while (1)
{
pthread_mutex_lock(&cc->mutex1);
++cc->sequence1;
sleep(1);
pthread_mutex_lock(&cc->mutex2);
++cc->sequence2;
pthread_mutex_unlock(&cc->mutex2);
pthread_mutex_unlock(&cc->mutex1);
}
}
void* thread2(void* arg)
{
Counter *cc = (Counter *)arg;
while (1)
{
pthread_mutex_lock(&cc->mutex2);
++cc->sequence2;
sleep(1);
pthread_mutex_lock(&cc->mutex1);
++cc->sequence1;
pthread_mutex_unlock(&cc->mutex1);
pthread_mutex_unlock(&cc->mutex2);
}
}
int main()
{
Counter pub_counter = {PTHREAD_MUTEX_INITIALIZER, PTHREAD_MUTEX_INITIALIZER, 0, 0};
pthread_t tid[2];
if (pthread_create(&tid[0], NULL, &thread1, &pub_counter) != 0)
{
_exit(1);
}
if (pthread_create(&tid[1], NULL, &thread2, &pub_counter) != 0)
{
_exit(1);
}
pthread_join(tid[0], NULL);
pthread_join(tid[1], NULL);
return 0;
}
- 2：编译运行
编译时加上-g选项，以便能够得到符号对应的源码
gcc -o deadlock -g deadlock.c -pthread
./deadlock
- 3：pstack查看调用栈
使用pstack命令，可以查看正在运行的进程的调用栈：
多运行几次，发现每次的打印中，线程2和3都卡在__lll_lock_wait函数中，这就是一个明显的死锁发生的信号了。
# ps -ef|grep deadlock
root 9867 9032 0 14:13 pts/7 00:00:00 ./deadlock
root 9956 8991 0 14:16 pts/5 00:00:00 grep --color=auto deadlock
# pstack 9867
Thread 3 (Thread 0x7f6093bf6700 (LWP 9868)):
#0 0x00007f6093fc61bd in __lll_lock_wait () from /lib64/libpthread.so.0
#1 0x00007f6093fc1d02 in _L_lock_791 () from /lib64/libpthread.so.0
#2 0x00007f6093fc1c08 in pthread_mutex_lock () from /lib64/libpthread.so.0
#3 0x00000000004007d8 in thread1 (arg=0x7fffad4cbeb0) at deadlock.c:26
#4 0x00007f6093fbfdc5 in start_thread () from /lib64/libpthread.so.0
#5 0x00007f6093cee76d in clone () from /lib64/libc.so.6
Thread 2 (Thread 0x7f60933f5700 (LWP 9869)):
#0 0x00007f6093fc61bd in __lll_lock_wait () from /lib64/libpthread.so.0
#1 0x00007f6093fc1d02 in _L_lock_791 () from /lib64/libpthread.so.0
#2 0x00007f6093fc1c08 in pthread_mutex_lock () from /lib64/libpthread.so.0
#3 0x0000000000400852 in thread2 (arg=0x7fffad4cbeb0) at deadlock.c:44
#4 0x00007f6093fbfdc5 in start_thread () from /lib64/libpthread.so.0
#5 0x00007f6093cee76d in clone () from /lib64/libc.so.6
Thread 1 (Thread 0x7f60943e1740 (LWP 9867)):
#0 0x00007f6093fc0ef7 in pthread_join () from /lib64/libpthread.so.0
#1 0x0000000000400908 in main () at deadlock.c:66
- 4.1 attach到进程：使用gdb命令，attach到进程上，查看锁的状态：
# gdb attach 9867
GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-94.el7
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
attach: No such file or directory.
Attaching to process 9867
Reading symbols from /root/devel/mycode/deadlock...done.
Reading symbols from /lib64/libpthread.so.0...(no debugging symbols found)...done.
[New LWP 9869]
[New LWP 9868]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Loaded symbols for /lib64/libpthread.so.0
Reading symbols from /lib64/libc.so.6...(no debugging symbols found)...done.
Loaded symbols for /lib64/libc.so.6
Reading symbols from /lib64/ld-linux-x86-64.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib64/ld-linux-x86-64.so.2
0x00007f6093fc0ef7 in pthread_join () from /lib64/libpthread.so.0
Missing separate debuginfos, use: debuginfo-install glibc-2.17-157.el7_3.4.x86_64
- 4.2查看改进程当前有哪些线程：(可以结合 info thr、thr apply all bt 等命令确定哪些线程在等锁。)
*说明当前正在线程1上，需要切换到线程2和线程3上，查看锁的状态。
(gdb) info thread
Id Target Id Frame
3 Thread 0x7f6093bf6700 (LWP 9868) "deadlock" 0x00007f6093fc61bd in __lll_lock_wait () from /lib64/libpthread.so.0
2 Thread 0x7f60933f5700 (LWP 9869) "deadlock" 0x00007f6093fc61bd in __lll_lock_wait () from /lib64/libpthread.so.0
* 1 Thread 0x7f60943e1740 (LWP 9867) "deadlock" 0x00007f6093fc0ef7 in pthread_join () from /lib64/libpthread.so.0
- 先切换到线程2上，并打印调用栈：
(gdb) thread 2
[Switching to thread 2 (Thread 0x7f60933f5700 (LWP 9869))]
#0 0x00007f6093fc61bd in __lll_lock_wait () from /lib64/libpthread.so.0
(gdb) bt
#0 0x00007f6093fc61bd in __lll_lock_wait () from /lib64/libpthread.so.0
#1 0x00007f6093fc1d02 in _L_lock_791 () from /lib64/libpthread.so.0
#2 0x00007f6093fc1c08 in pthread_mutex_lock () from /lib64/libpthread.so.0
#3 0x0000000000400852 in thread2 (arg=0x7fffad4cbeb0) at deadlock.c:44
#4 0x00007f6093fbfdc5 in start_thread () from /lib64/libpthread.so.0
#5 0x00007f6093cee76d in clone () from /lib64/libc.so.6
- 线程2的”PID”为9869。
调用栈显示该线程正阻塞在pthread_mutex_lock上。尝试看一下锁的状态：
(gdb) p cc
No symbol "cc" in current context.
(gdb) frame 3
#3 0x0000000000400852 in thread2 (arg=0x7fffad4cbeb0) at deadlock.c:44
44 pthread_mutex_lock(&cc->mutex1);
(gdb) p cc
$1 = (Counter *) 0x7fffad4cbeb0
(gdb) p cc->mutex1
$2 = {__data = {__lock = 2, __count = 0, __owner = 9868, __nusers = 1, __kind = 0, __spins = 0, __list = {__prev = 0x0,
__next = 0x0}}, __size = "\002\000\000\000\000\000\000\000\214&\000\000\001", '\000' <repeats 26 times>, __align = 2}
(gdb) p cc->mutex2
$3 = {__data = {__lock = 2, __count = 0, __owner = 9869, __nusers = 1, __kind = 0, __spins = 0, __list = {__prev = 0x0,
__next = 0x0}}, __size = "\002\000\000\000\000\000\000\000\215&\000\000\001", '\000' <repeats 26 times>, __align = 2}
(gdb) p cc->sequence1
$4 = 1
(gdb) p cc->sequence2
$5 = 1
因为当前正处于栈帧0上，也就是__lll_lock_wait函数中，因此尝试打印cc时，会报：No symbol "cc" in current context。
因此，首先需要使用frame 3命令，切换到调用pthread_mutex_lock之前的栈帧，然后打印出cc中的各个属性。
可见，cc->mutex1当前被”PID”为9868的线程所持有，而cc->mutex2被”PID”为9869的线程，也就是当前线程所持有。
- 然后，切换到线程3上，然后查看调用栈以及锁的状态：
可见，线程3的”PID”为9868，它就是持有cc->mutex1的线程，而该线程所请求lock的cc->mutex2，目前正被”PID”为9869的线程持有，也就是线程2。
(gdb) thread 3
[Switching to thread 3 (Thread 0x7f6093bf6700 (LWP 9868))]
#0 0x00007f6093fc61bd in __lll_lock_wait () from /lib64/libpthread.so.0
(gdb) bt
#0 0x00007f6093fc61bd in __lll_lock_wait () from /lib64/libpthread.so.0
#1 0x00007f6093fc1d02 in _L_lock_791 () from /lib64/libpthread.so.0
#2 0x00007f6093fc1c08 in pthread_mutex_lock () from /lib64/libpthread.so.0
#3 0x00000000004007d8 in thread1 (arg=0x7fffad4cbeb0) at deadlock.c:26
#4 0x00007f6093fbfdc5 in start_thread () from /lib64/libpthread.so.0
#5 0x00007f6093cee76d in clone () from /lib64/libc.so.6
(gdb) f 3
#3 0x00000000004007d8 in thread1 (arg=0x7fffad4cbeb0) at deadlock.c:26
26 pthread_mutex_lock(&cc->mutex2);
(gdb) p cc->mutex1
$7 = {__data = {__lock = 2, __count = 0, __owner = 9868, __nusers = 1, __kind = 0, __spins = 0, __list = {__prev = 0x0,
__next = 0x0}}, __size = "\002\000\000\000\000\000\000\000\214&\000\000\001", '\000' <repeats 26 times>, __align = 2}
(gdb) p cc->mutex2
$8 = {__data = {__lock = 2, __count = 0, __owner = 9869, __nusers = 1, __kind = 0, __spins = 0, __list = {__prev = 0x0,
__next = 0x0}}, __size = "\002\000\000\000\000\000\000\000\215&\000\000\001", '\000' <repeats 26 times>, __align = 2}
(gdb) p cc->sequence1
$9 = 1
(gdb) p cc->sequence2
$10 = 1
Owner 字段表示是哪个线程持有这把锁，它是线程的 LWP 号，可以通过 info thr 查看。
- 5：附注
当使用GDB调试进程时，如果该进程收到了信号，对于不同的信号，GDB会有不同的动作。
（1）有些信号会使得GDB将进程停住，或者直接将信号传递给进程。
（2）可以使用info signals或info handle命令，查看GDB收到信号时采取的动作：
（3）可见，对于SIGINT和SIGTRAP信号，默认情况下GDB会停止进程的运行，并且不将信号传递给进程。因此，可以利用这两个信号，暂停进程的运行，打印调试信息，然后使用continue命令，使进程继续运行。
(gdb) info signals
Signal Stop Print Pass to program Description
SIGHUP Yes Yes Yes Hangup
SIGINT Yes Yes No Interrupt
SIGQUIT Yes Yes Yes Quit
SIGILL Yes Yes Yes Illegal instruction
SIGTRAP Yes Yes No Trace/breakpoint trap
SIGABRT Yes Yes Yes Aborted
SIGEMT Yes Yes Yes Emulation trap
SIGFPE Yes Yes Yes Arithmetic exception
SIGKILL Yes Yes Yes Killed
…
参考：《GDB/Debug.Hacks中文版：深入调试的技术和工具》，使用pstack和gdb调试死锁，Linux开发调试经验整理(一)——Release版本死锁定位
