检查出现死锁的运行

下面列出了导致实际死锁的哲学家就餐程序的一次运行。

% cc -g -o din_philo din_philo.c
% collect -r deadlock -o din_philo.1.er din_philo
Creating experiment database din_philo.1.er ...
Philosopher 1 is done thinking and now ready to eat.
Philosopher 2 is done thinking and now ready to eat.
Philosopher 3 is done thinking and now ready to eat.
Philosopher 0 is done thinking and now ready to eat.
Philosopher 1: got right  chopstick 1
Philosopher 3: got right  chopstick 3
Philosopher 0: got right  chopstick 0
Philosopher 1: got left chopstick 2
Philosopher 3: got left chopstick 4
Philosopher 4 is done thinking and now ready to eat.
Philosopher 1: eating.
Philosopher 3: eating.
Philosopher 3: got right  chopstick 3
Philosopher 4: got right  chopstick 4
Philosopher 2: got right  chopstick 2
Philosopher 0: got left chopstick 1
Philosopher 0: eating.
Philosopher 1: got right  chopstick 1
Philosopher 4: got left chopstick 0
Philosopher 4: eating.
Philosopher 0: got right  chopstick 0
Philosopher 3: got left chopstick 4
Philosopher 3: eating.
Philosopher 4: got right  chopstick 4
Philosopher 2: got left chopstick 3
Philosopher 2: eating.
Philosopher 3: got right  chopstick 3
Philosopher 1: got left chopstick 2
Philosopher 1: eating.
Philosopher 2: got right  chopstick 2
Philosopher 0: got left chopstick 1
Philosopher 0: eating.
Philosopher 1: got right  chopstick 1
Philosopher 4: got left chopstick 0
Philosopher 4: eating.
Philosopher 0: got right  chopstick 0
Philosopher 3: got left chopstick 4
Philosopher 3: eating.
...
Philosopher 4: got right  chopstick 4
Philosopher 2: got left chopstick 3
Philosopher 2: eating.
Philosopher 2: got right  chopstick 2
Philosopher 3: got right  chopstick 3
(hang)

Execution terminated by pressing CTRL-C

键入以下命令以使用 er_print 实用程序检查实验：

% er_print din_philo.1.er
(er_print) deadlocks

Deadlock #1, Potential deadlock
        Thread #2
                Lock being held:        0x21380, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
                Lock being requested:   0x21398, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
        Thread #3
                Lock being held:        0x21398, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
                Lock being requested:   0x213b0, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
        Thread #4
                Lock being held:        0x213b0, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
                Lock being requested:   0x213c8, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
        Thread #5
                Lock being held:        0x213c8, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
                Lock being requested:   0x213e0, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
        Thread #6
                Lock being held:        0x213e0, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
                Lock being requested:   0x21380, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"

Deadlock #2, Actual deadlock
        Thread #2
                Lock being held:        0x21380, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
                Lock being requested:   0x21398, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
        Thread #3
                Lock being held:        0x21398, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
                Lock being requested:   0x213b0, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
        Thread #4
                Lock being held:        0x213b0, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
                Lock being requested:   0x213c8, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
        Thread #5
                Lock being held:        0x213c8, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
                Lock being requested:   0x213e0, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
        Thread #6
                Lock being held:        0x213e0, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"
                Lock being requested:   0x21380, at: grab_chopstick + 0x00000024, line 105 in "din_philo.c"

以下屏幕抓图显示了线程分析器中显示的死锁信息。

图 3-2  din_philo.c 中检测到的死锁

线程分析器报告了 din_philo.c 的两个死锁，一个是潜在死锁，另一个是实际死锁。通过进一步检查，可以发现两个死锁是相同的。

该死锁涉及的循环链如下：

线程 2：在地址 0x21380 持有锁，在地址 0x21398 请求锁 线程 3：在地址 0x21398 持有锁，在地址 0x213b0 请求锁 线程 4：在地址 0x213b0 持有锁，在地址 0x213c8 请求锁 线程 5：在地址 0x213c8 持有锁，在地址 0x213e0 请求锁 线程 6：在地址 0x213e0 持有锁，在地址 0x21380 请求锁

选择循环链中的第一个线程（线程 2），然后单击 "Dual Source"（双源）视图，可以在源代码中看到，线程 2 获得了地址 0x21380 处的锁，在源代码中还可以看到，该线程请求了地址 0x21398 处的锁。

以下屏幕抓图显示了对应于线程 2 的 "Dual Source"（双源）视图。屏幕抓图的上半部分显示了线程 2 通过在第 105 行调用 pthread_mutex_lock() 获取了地址 0x21380 处的锁。屏幕抓图的下半部分显示了同一线程通过在第 105 行调用 pthread_mutex_lock() 请求了地址 0x21398 处的锁。在对 pthread_mutex_lock() 的两次调用中，分别使用了不同的锁作为参数。通常，锁获取和锁请求操作可能不在同一源代码行上。

在屏幕抓图中每个源代码行的左侧会显示缺省的度量："Exclusive Deadlocks"（互斥死锁）度量。此度量显示了该源代码行上报告的（死锁涉及的）锁获取或锁请求操作的总次数。只有属于死锁循环链一部分的源代码行才具有此度量的值，并且该值大于零。

图 3-3  din_philo.c 中的潜在死锁