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Oracle® Developer Studio 12.6: Thread Analyzer User's Guide

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Updated: June 2017
 
 

Understanding the Deadlock Experiment Results

This section explains how to use Thread Analyzer to investigate the deadlocks in the dining philosopher program.

Examining Runs That Deadlock

The following listing shows a run of the dining philosophers program that results in an actual deadlock.

% 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

Type the following commands to examine the experiment with er_print utility:

% er_print din_philo.1.er
(er_print) deadlocks
Deadlock #1, Potential deadlock        
     Thread #2
          Lock being held: 0x21300, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
          Lock being requested: 0x21318, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Thread #3
          Lock being held: 0x21318, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
          Lock being requested: 0x21330, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Thread #4
          Lock being held: 0x21330, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
          Lock being requested: 0x21348, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Thread #5
          Lock being held: 0x21348, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
          Lock being requested: 0x21360, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Thread #6
          Lock being held: 0x21360, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
          Lock being requested: 0x21300, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
    
Deadlock #2, Actual deadlock
     Thread #2 
          Lock being held: 0x21300, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
          Lock being requested: 0x21318, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Thread #3
          Lock being held: 0x21318, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
          Lock being requested: 0x21330, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Thread #4
          Lock being held: 0x21330, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
          Lock being requested: 0x21348, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Thread #5
          Lock being held: 0x21348, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
          Lock being requested: 0x21360, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Thread #6
          Lock being held: 0x21360, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
          Lock being requested: 0x21300, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"

Deadlocks List Summary: Experiment: din_philo.1.er
Total Deadlocks: 2
(er_print)

The following screen shot shows Thread Analyzer's presentation of the deadlock information.

Figure 7  Deadlock Detected in din_philo.c

image:A screen shot of the Thread Analyzer's window, which shows an actual deadlock.

Thread Analyzer reports two deadlocks for din_philo.c, one potential and the other actual. On closer inspection, you find that the two deadlocks are identical.

The circular chain involved in the deadlock is as follows:

Thread 2: holds lock at address 0x21300, requests lock at address 0x21318
Thread 3: holds lock at address 0x21318, requests lock at address 0x21330
Thread 4: holds lock at address 0x21330, requests lock at address 0x21348
Thread 5: holds lock at address 0x21348, requests lock at address 0x21360
Thread 6: holds lock at address 0x21360, requests lock at address 0x21300

Select the first thread in the chain (Thread #2) and then click the Dual Source view to see the source code location in which Thread #2 held the lock at address 0x21430, and where in the source code it requested the lock at address 0x21448. The following figure shows the Dual Source view for Thread #2.

The following screen shot shows the Dual Source view for Thread #2. The top half of the screen shot shows that Thread #2 acquired the lock at address 0x21300 by calling pthread_mutex_lock() on line 105. The bottom half of the screen shot shows that the same thread requested the lock at address 0x21318 by calling pthread_mutex_lock on line 105. Each of the two calls to pthread_mutex_lock used a different lock as the argument. In general, the lock-acquire and lock-request operations might not be on the same source line.

The default metric (Exclusive Deadlocks metric) is shown to the left of each source line in the screen shot. This metric gives a count of the number of times a lock-acquire or lock-request operation, which was involved in a deadlock, was reported on that source line. Only source lines that are part of a deadlock chain would have a value for this metric that is larger than zero.

Figure 8  Potential Deadlock in din_philo.c

image:A screen shot of the Thread Analyzer's Dual-Source tab which shows a potential deadlock.

Examining Runs That Complete Despite Deadlock Potential

The dining philosophers program can avoid actual deadlock and terminate normally if you supply a large enough sleep argument. Normal termination, however, does not mean the program is safe from deadlocks. It simply means that the locks that were held and requested did not form a deadlock chain during a given run. If the timing changes in other runs, an actual deadlock can occur. The following listing shows a run of the dining philosophers program that terminates normally because of the 40 second sleep time (supplied as an argument to the executable).

% cc -g -o din_philo_pt din_philo.c
% collect -r deadlock -o din_philo_pt.1.er din_philo_pt 40
Creating experiment database tha.2.er ...
Philosopher 0 is done thinking and now ready to eat.
Philosopher 2 is done thinking and now ready to eat.
Philosopher 1 is done thinking and now ready to eat.
Philosopher 3 is done thinking and now ready to eat.
Philosopher 2: got right  chopstick 2
Philosopher 3: got right  chopstick 3
Philosopher 0: got right  chopstick 0
Philosopher 4 is done thinking and now ready to eat.
Philosopher 0: got left chopstick 1
Philosopher 0: eating.
Philosopher 3: got left chopstick 4
Philosopher 3: eating.
Philosopher 0: got left chopstick 1
Philosopher 0: eating.
Philosopher 0: got right  chopstick 0
Philosopher 2: got left chopstick 3
Philosopher 2: eating.
...
Philosopher 4: got right  chopstick 4
Philosopher 3: got right  chopstick 3
Philosopher 2: got right  chopstick 2
Philosopher 4: got left chopstick 0
Philosopher 4: eating.
Philosopher 4 is done eating.
Philosopher 3: got left chopstick 4
Philosopher 3: eating.
Philosopher 0: got right  chopstick 0
Philosopher 0: got left chopstick 1
Philosopher 0: eating.
Philosopher 3 is done eating.
Philosopher 2: got left chopstick 3
Philosopher 2: eating.
Philosopher 0 is done eating.
Philosopher 2 is done eating.
Philosopher 1: got right  chopstick 1
Philosopher 1: got left chopstick 2
Philosopher 1: eating.
Philosopher 1 is done eating.
%

Execution terminated normally

Type the following commands shown at the prompts to examine the experiment with er_print utility:

% er_print din_philo_pt.1.er
(er_print) deadlocks
Deadlock #1, Potential deadlock
    
Thread #2
     Lock being held: 0x21300, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Lock being requested: 0x21318, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
Thread #3
     Lock being held: 0x21318, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Lock being requested: 0x21330, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
Thread #4
     Lock being held: 0x21330, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Lock being requested: 0x21348, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
Thread #5
     Lock being held: 0x21348, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Lock being requested: 0x21360, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
Thread #6
     Lock being held: 0x21360, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"
     Lock being requested: 0x21300, at: grab_chopstick + 0x0000003C, line 105 in "din_philo.c"

Deadlocks List Summary: Experiment: din_philo_pt.1.er/ Total Deadlocks: 1
(er_print)

The following screen shot shows the potential deadlock information in Thread Analyzer.

Figure 9  Potential deadlock in din_philo.c

image:A screen shot of the Thread Analyzer window which shows a potential deadlock.

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