Re: pthread_mutex_lock hangs on unlocked mutex

[Ian Kent]

On Fri, 7 Nov 2008, Michael Dressel wrote:

> Hi,
> 
> (I'm not subscribed to the list, please CC me.)
> 
> in our software three processes are using several pthread mutexes.
> Sometimes a process hangs inside pthread_mutex_lock even though the mutex is
> not locked. I can tell it's not locked because another process is still
> running and locking and unlocking the mutex.

pthreads is implemented in glibc.
If you really think there is a bug in the ptheads implementation then 
the glibc maintainers will require you to produce a simple example program 
which demonstrates the bug before it's accepted as a bug.

When you say processes you mean threads, right?

If you can't produce such an example program and you can you prove (to 
yourself) there are no use after free or execution order issues with your 
code then your only option is to develop a workaround.

You code wouldn't happen to be doing thread synchronization with 
pthread_cond_wait()/pthread_cond_signal() would it?

> When I connect with gdb to the hanging process I find:
> 
> #0  0xffffe430 in __kernel_vsyscall ()
> #1  0xf7d5e7a9 in __lll_lock_wait () from /lib/libpthread.so.0
> #2  0xf7d59c75 in _L_lock_288 () from /lib/libpthread.so.0
> #3  0xf7d596c5 in pthread_mutex_lock () from /lib/libpthread.so.0
> #4  0xf7e444b6 in pthread_mutex_lock () from /lib/libc.so.6
> #5  0x080fb338 in <myfunction> (mutexP=0x70f4165c)
> 
> And the mutexP looks like this:
> $1 = {__data = {__lock = 0, __count = 0, __owner = 0, __kind = 1,
>     __nusers = 0, {__spins = 0, __list = {__next = 0x0}}},
>   __size = '\0' <repeats 12 times>,
> "\001\000\000\000\000\000\000\000\000\000\00
> 0", __align = 0}
> 
> I guess the process had to wait for the mutex but when the mutex was unlocked
> the signal to the waiting process got lost.
> 
> Our software was build on a SUUSE 10.1
> 	kernel 2.6.16.13-4-default
> 	glibc-32bit-2.4-27
> system as 32 bit binary. On This system the problem does not occur.
> 
> The system showing the problem is a dual processor SUSE 11.0
> 	kernel 2.6.25.18-0.2-default
> 	glibc-32bit-2.8-14.1
> system. Also if the second core is disabled in the BIOS we have the
> problem. Some more details of the system are listed below.
> 
> When the system hangs it does not recover from that situation. But if
> I send a kill -STOP <pid>; kill -CONT <pid> sequence to the hanging
> process it does continue.
> The same effect is reached by connecting and disconnecting with gdb.
> 
> Is there any way, either in configuring the system or pthread, to prevent
> this problem?
> 
> Cheers,
> Michael
> 
> openSUSE 11.0 (X86-64)
> 
> Linux 2.6.25.18-0.2-default #1 SMP 2008-10-21 16:30:26 +0200 x86_64 x86_64
> x86_64 GNU/Linux
> 
> glibc-32bit-2.8-14.1
> 
> processor       : 0
> vendor_id       : GenuineIntel
> cpu family      : 6
> model           : 15
> model name      : Intel(R) Core(TM)2 Duo CPU     E6550  @ 2.33GHz
> stepping        : 11
> cpu MHz         : 1998.000
> cache size      : 4096 KB
> physical id     : 0
> siblings        : 1
> core id         : 0
> cpu cores       : 1
> fpu             : yes
> fpu_exception   : yes
> cpuid level     : 10
> wp              : yes
> flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca
> cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm
> constant_tsc arch_perfmon pebs bts rep_good pni monitor ds_cpl vmx smx est tm2
> ssse3 cx16 xtpr lahf_lm
> bogomips        : 4658.50
> clflush size    : 64
> cache_alignment : 64
> address sizes   : 36 bits physical, 48 bits virtual
> power management:
> 
> 
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