? ? ? ? 在《DllMain中不當操作導致死鎖問題的分析--死鎖介紹》一文中，我們介紹了死鎖產生的原因。一般來說，如果我們對線程同步技術掌握不牢，或者同步方案混亂，極容易導致死鎖。本文我們將介紹如何使用valgrind排查死鎖問題。（轉載請指明出于breaksoftware的csdn博客）

? ? ? ? 構造一個場景

#include <pthread.h>pthread_mutex_t s_mutex_a;
pthread_mutex_t s_mutex_b;
pthread_barrier_t s_barrier;void lock() {pthread_mutex_lock(&s_mutex_b);{pthread_barrier_wait(&s_barrier);pthread_mutex_lock(&s_mutex_a);pthread_mutex_unlock(&s_mutex_a);}pthread_mutex_unlock(&s_mutex_b);
}static void* thread_routine(void* arg) {pthread_mutex_lock(&s_mutex_a);{pthread_barrier_wait(&s_barrier);pthread_mutex_lock(&s_mutex_b);pthread_mutex_unlock(&s_mutex_b);}pthread_mutex_unlock(&s_mutex_a);
}int main(int argc, char** argv) {pthread_t tid;pthread_mutex_init(&s_mutex_a, 0);pthread_mutex_init(&s_mutex_b, 0);pthread_barrier_init(&s_barrier, 0, 2);pthread_create(&tid, 0, &thread_routine, 0);lock();pthread_join(tid, 0);pthread_cancel(tid);pthread_barrier_destroy(&s_barrier);pthread_mutex_destroy(&s_mutex_a);pthread_mutex_destroy(&s_mutex_b);return 0;
}

? ? ? ? 這段代碼我們只要關注lock和thread_routine兩個方法。

? ? ? ? lock方法在主線程中執行，它先給s_mutex_b上鎖，然后通過屏障s_barrier等待線程也執行到屏障處（第21行）。

? ? ? ? thread_routine是線程函數，它先給s_mutex_a上鎖，然后通過屏障s_barrier等待主線程也執行到屏障處（第10行）。

? ? ? ? 主線程和子線程都執行到屏障處后，屏障被打開，它們繼續向下執行：主線程執行到第12行試圖獲取s_mutex_a；子線程執行到第23行試圖獲取s_mutex_b。由于這兩個互斥量已經被占用，所以產生死鎖。

? ? ? ? 這是通過代碼分析出來的，但是對于比較大的工程項目，我們則需要通過工具來分析。下面我們使用valgrind來分析

valgrind --tool=drd --trace-mutex=yes ./dead_lock

? ? ? ? 我們使用上面指令，讓valgrind把互斥量相關的信息給打印出來

==4749== [1] mutex_init      mutex 0x30a040
==4749== [1] mutex_init      mutex 0x30a0a0
==4749== [1] mutex_init      mutex 0x1ffefffe10
==4749== [1] mutex_ignore_ordering mutex 0x1ffefffe10
==4749== [1] mutex_trylock   mutex 0x1ffefffe10 rc 0 owner 0
==4749== [1] post_mutex_lock mutex 0x1ffefffe10 rc 0 owner 0
==4749== [1] mutex_unlock    mutex 0x1ffefffe10 rc 1
==4749== [2] mutex_trylock   mutex 0x1ffefffe10 rc 0 owner 1
==4749== [2] post_mutex_lock mutex 0x1ffefffe10 rc 0 owner 1
==4749== [2] mutex_unlock    mutex 0x1ffefffe10 rc 1
==4749== [2] mutex_trylock   mutex 0x30a040 rc 0 owner 0
==4749== [2] post_mutex_lock mutex 0x30a040 rc 0 owner 0
==4749== [1] cond_post_wait  mutex 0x1ffefffe10 rc 0 owner 2
==4749== [1] mutex_unlock    mutex 0x1ffefffe10 rc 1
==4749== [1] mutex_destroy   mutex 0x1ffefffe10 rc 0 owner 1
==4749== [1] mutex_trylock   mutex 0x30a0a0 rc 0 owner 0
==4749== [1] post_mutex_lock mutex 0x30a0a0 rc 0 owner 0
==4749== [1] mutex_trylock   mutex 0x30a040 rc 1 owner 2
==4749== [2] mutex_trylock   mutex 0x30a0a0 rc 1 owner 1

? ? ? ? 第18行顯示線程1試圖給0x30a040互斥量上鎖，但是該互斥量的所有者（owner）是線程2。

? ? ? ? 第19行顯示線程2試圖給0x30a0a0互斥量上鎖，但是該互斥量的所有者（owner）是線程1。

? ? ? ? 如此我們便可以確定這段程序卡住是因為死鎖導致的。

? ? ? ? 但是DRD有個問題，不能指出發生死鎖的位置。這個時候Helgrind該出場了。

valgrind --tool=helgrind ./dead_lock 

? ? ? ? helgrind執行時，如果發生死鎖，需要ctrl+C來終止運行，于是可以得到如下結果

==5373== Process terminating with default action of signal 2 (SIGINT)
==5373==    at 0x4E5310D: __lll_lock_wait (lowlevellock.S:135)
==5373==    by 0x4E4C022: pthread_mutex_lock (pthread_mutex_lock.c:78)
==5373==    by 0x4C33FD6: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==5373==    by 0x108A11: lock (dead_lock.c:12)
==5373==    by 0x108AF4: main (dead_lock.c:38)
==5373== ---Thread-Announcement------------------------------------------
==5373== 
==5373== Thread #2 was created
==5373==    at 0x518287E: clone (clone.S:71)
==5373==    by 0x4E49EC4: create_thread (createthread.c:100)
==5373==    by 0x4E49EC4: pthread_create@@GLIBC_2.2.5 (pthread_create.c:797)
==5373==    by 0x4C36A27: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==5373==    by 0x108AEA: main (dead_lock.c:36)
==5373== 
==5373== ----------------------------------------------------------------
==5373== 
==5373== Thread #2: Exiting thread still holds 1 lock
==5373==    at 0x4E5310D: __lll_lock_wait (lowlevellock.S:135)
==5373==    by 0x4E4C022: pthread_mutex_lock (pthread_mutex_lock.c:78)
==5373==    by 0x4C33FD6: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==5373==    by 0x108A5C: thread_routine (dead_lock.c:23)
==5373==    by 0x4C36C26: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==5373==    by 0x4E496DA: start_thread (pthread_create.c:463)
==5373==    by 0x518288E: clone (clone.S:95)
==5373== 
==5373== ---Thread-Announcement------------------------------------------
==5373== 
==5373== Thread #1 is the program's root thread
==5373== 
==5373== ----------------------------------------------------------------
==5373== 
==5373== Thread #1: Exiting thread still holds 1 lock
==5373==    at 0x4E5310D: __lll_lock_wait (lowlevellock.S:135)
==5373==    by 0x4E4C022: pthread_mutex_lock (pthread_mutex_lock.c:78)
==5373==    by 0x4C33FD6: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==5373==    by 0x108A11: lock (dead_lock.c:12)
==5373==    by 0x108AF4: main (dead_lock.c:38)

? ? ? ? 第22和37行分別顯示子線程和主線程在中斷之前，都鎖在哪行，這樣就更容易定位問題了。

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