Linux on Power 上的调试工具和技术（5）

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清单 22.  查看每个线程运行到了什么地方

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(gdb) thread apply 1 where Thread 1 (Thread 1073991712 (LWP 9510)): #0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6 #1  0x0ff4ac28 in __write_nocancel () from /lib/tls/libc.so.6 Previous frame identical to this frame (corrupt stack?) #0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6 (gdb) thread apply 2 where Thread 2 (Thread 1090771744 (LWP 9514)): #0  0x0ffe94ec in __lll_lock_wait () from /lib/tls/libpthread.so.0 #1  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #2  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #3  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #4  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 Previous frame inner to this frame (corrupt stack?) #0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6 (gdb) thread apply 3 where Thread 3 (Thread 1086577440 (LWP 9513)): #0  0x0ffe94ec in __lll_lock_wait () from /lib/tls/libpthread.so.0 #1  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #2  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #3  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #4  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 Previous frame inner to this frame (corrupt stack?) #0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6 (gdb) thread apply 4 where Thread 4 (Thread 1082383136 (LWP 9512)): #0  0x0ffe94ec in __lll_lock_wait () from /lib/tls/libpthread.so.0 #1  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #2  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #3  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #4  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 Previous frame inner to this frame (corrupt stack?) #0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6 (gdb) thread apply 5 where Thread 5 (Thread 1078188832 (LWP 9511)): #0  0x0ffe94ec in __lll_lock_wait () from /lib/tls/libpthread.so.0 #1  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #2  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #3  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 #4  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0 Previous frame inner to this frame (corrupt stack?) #0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6

每个线程都试图对一个互斥体进行加锁，但是这个互斥体却是不可用的，可能是因为有另外一个线程已经对其进行加锁了。从上面的证据我们可以判断程序中一定存在死锁。您还可以看到哪个线程现在拥有这个互斥体。
清单 23. 查看哪个线程拥有互斥体

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(gdb) print AccountA_mutex $1 = {__m_reserved = 2, __m_count = 0, __m_owner = 0x2527, <br> __m_kind = 0, __m_lock  = {__status = 1, __spinlock = 0}} (gdb) print 0x2527 $2 = 9511 (gdb) print AccountB_mutex $3 = {__m_reserved = 2, __m_count = 0, __m_owner = 0x2529, <br> __m_kind = 0, __m_lock = {__status = 1, __spinlock = 0}} (gdb) print 0x2529 $4 = 9513 (gdb)

从上面的命令中，我们可以看出 AccontA_mutex 是被线程 5（LWP 9511）加锁（拥有）的，而 AccontB_mutex 是被线程 3（LWP 9513）加锁（拥有）的。
为了解决上面的死锁情况，可以按照相同的顺序对互斥体进行加锁，如下所示：
清单 24. 按照相同的顺序对互斥体进行加锁

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. . void * transferAB (void* amount_ptr) {         int amount = *((int*)amount_ptr);         pthread_mutex_lock(&AccountA_mutex);         pthread_mutex_lock(&AccountB_mutex);         if (accountA.balance < amount)   {                 printf("There is not enough memory in Account A!\n");                 pthread_mutex_unlock(&AccountA_mutex);                 pthread_exit((void *)1);         }         accountA.balance -=amount;         sleep(1);         accountB.balance +=amount;         pthread_mutex_unlock(&AccountA_mutex);         pthread_mutex_unlock(&AccountB_mutex); } void * transferBA (void* amount_ptr) {         int amount = *((int*)amount_ptr);         pthread_mutex_lock(&AccountA_mutex);         pthread_mutex_lock(&AccountB_mutex);         if (accountB.balance < amount)   {                 printf("There is not enough memory in Account B!\n");                 pthread_mutex_unlock(&AccountB_mutex);                 pthread_exit((void *)1);         }         accountB.balance -=amount;         sleep(1);         accountA.balance +=amount;         pthread_mutex_unlock(&AccountA_mutex);         pthread_mutex_unlock(&AccountB_mutex); } . .

或者对每个帐号单独进行加锁，如下所示：
清单 25. 对每个帐号单独进行加锁

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. . void * transferAB (void* amount_ptr) {         int amount = *((int*)amount_ptr);         pthread_mutex_lock(&AccountA_mutex);         if (accountA.balance < amount)   {                 printf("There is not enough memory in Account A!\n");                 pthread_mutex_unlock(&AccountA_mutex);                 pthread_exit((void *)1);         }         accountA.balance -=amount;         sleep(1);         pthread_mutex_unlock(&AccountA_mutex);         pthread_mutex_lock(&AccountB_mutex);         accountB.balance +=amount;         pthread_mutex_unlock(&AccountB_mutex); } void * transferBA (void* amount_ptr) {         int amount = *((int*)amount_ptr);         pthread_mutex_lock(&AccountB_mutex);         if (accountB.balance < amount)   {                 printf("There is not enough memory in Account B!\n");                 pthread_mutex_unlock(&AccountB_mutex);                 pthread_exit((void *)1);         }         accountB.balance -=amount;         sleep(1);         pthread_mutex_unlock(&AccountB_mutex);         pthread_mutex_lock(&AccountA_mutex);         accountA.balance +=amount;         pthread_mutex_unlock(&AccountA_mutex); } . . .

要调试 64 位的应用程序（使用 GCC 的 –m64 选项或 IBM XL C/C++ 编译器的  –q64 选项编译），应该使用一个特别的 gdb 版本 gdb64。