Is it possible to determine the thread holding a mutex?

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You can use knowledge of the mutex internals to do this. Ordinarily this wouldn’t be a very good idea, but it’s fine for debugging.

Under Linux with the NPTL implementation of pthreads (which is any modern glibc), you can examine the __data.__owner member of the pthread_mutex_t structure to find out the thread that currently has it locked. This is how to do it after attaching to the process with gdb:

(gdb) thread 2
[Switching to thread 2 (Thread 0xb6d94b90 (LWP 22026))]#0  0xb771f424 in __kernel_vsyscall ()
(gdb) bt
#0  0xb771f424 in __kernel_vsyscall ()
#1  0xb76fec99 in __lll_lock_wait () from /lib/i686/cmov/libpthread.so.0
#2  0xb76fa0c4 in _L_lock_89 () from /lib/i686/cmov/libpthread.so.0
#3  0xb76f99f2 in pthread_mutex_lock () from /lib/i686/cmov/libpthread.so.0
#4  0x080484a6 in thread (x=0x0) at mutex_owner.c:8
#5  0xb76f84c0 in start_thread () from /lib/i686/cmov/libpthread.so.0
#6  0xb767784e in clone () from /lib/i686/cmov/libc.so.6
(gdb) up 4
#4  0x080484a6 in thread (x=0x0) at mutex_owner.c:8
8               pthread_mutex_lock(&mutex);
(gdb) print mutex.__data.__owner
$1 = 22025
(gdb)

(I switch to the hung thread; do a backtrace to find the pthread_mutex_lock() it’s stuck on; change stack frames to find out the name of the mutex that it’s trying to lock; then print the owner of that mutex). This tells me that the thread with LWP ID 22025 is the culprit.

You can then use thread find 22025 to find out the gdb thread number for that thread and switch to it.

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