Re: Need help to interpret pthread_mutex_t structure as printed by gdb

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 15/08/2016 18:14, Olle Blomgren wrote:
>> Could you provide the glibc version, a complete backtrace of both processes
>> and if possible a testcase?
> 
> I use glibc-2.23.
> I have downloaded the source and built it with debug symbols. Full
> back traces follows below.
> Client back trace + printout of offending mutex. (This is the one that hangs)
> -------------------------------------------------------------------------------------------------------------------
> Thread 3 (Thread 0x7ff8d1fcc700 (LWP 14005)):
> #0  0x00007ff8d231b82d in poll () at ../sysdeps/unix/syscall-template.S:84
> #1  0x00007ff8d2b39434 in _daemon_monitor (attr=0x7ff8d2da8300
> <_mon_thr_attr>) at mam.c:1346
> #2  0x00007ff8d2db03e4 in start_thread (arg=0x7ff8d1fcc700) at
> pthread_create.c:333
> #3  0x00007ff8d23248ad in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
> 
> Thread 2 (Thread 0x7ff8d15cb700 (LWP 14006)):
> #0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
> #1  0x00007ff8d2db2a75 in __GI___pthread_mutex_lock
> (mutex=0x7ff8d16ac810) at ../nptl/pthread_mutex_lock.c:80
> #2  0x00007ff8d2b1f962 in lch_lock (name=919552, type=LCH_EXCL,
> tx=0x7ff8d3446080, ctx=0x7ff8c4001110)
>     at lch.c:124
> #3  0x00007ff8d2b40c1c in _get_page_get_entry (entry=0x7ff8d15cab80,
> nbr_bytes=8, ctx=0x7ff8c4001110)
>     at pge.c:1194
> #4  0x00007ff8d2b41b36 in pge_alloc_entry (entry=0x7ff8d15cab80,
> nbr_bytes=8, page_sz=1 '\001',
>     ctx=0x7ff8c4001110) at pge.c:1416
> #5  0x00007ff8d2b5d0bf in api_int_setup_entry
> (entry_arr=0x7ff8d15cacb8, page_ptr=0x7ff8d15cac88,
>     page_ref=0x7ff8d15cac80, lr=0x7ff8d15cad50, meta=0xf68d50,
> tx=0x7ff8d3446080, nbr_entries=1, sct=0,
>     ctx=0x7ff8c4001110) at api_int.c:47
> #6  0x00007ff8d2b5b410 in hope_new (tx=0x7ff8c4000cd0,
> elm_type=0x40b1c9 "NoColl", nv_list=0x40adf9 "tjing")
>     at hope.c:2495
> #7  0x0000000000405d6d in _new1 (attr=0x0) at api_tests.c:1579
> #8  0x00007ff8d2db03e4 in start_thread (arg=0x7ff8d15cb700) at
> pthread_create.c:333
> #9  0x00007ff8d23248ad in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
> 
> Thread 1 (Thread 0x7ff8d34e0700 (LWP 14004)):
> #0  0x00007ff8d2db164d in pthread_join (threadid=140706641131264,
> thread_return=0x0) at pthread_join.c:90
> #1  0x0000000000406252 in _scan_1new40000_1read_1lockmod5remove_3thr
> (tc=30, print_desc_only=0)
>     at api_tests.c:1682
> #2  0x000000000040a1b5 in _all_test_cases (tc=30, nbr_updates=0,
> start_value=0, name=0x0,
>     ref=18446744073709551615, print_desc_only=0) at api_tests.c:2981
> #3  0x000000000040a784 in main (argc=4, argv=0x7fffed178988) at api_tests.c:3084
> (gdb) t 2
> [Switching to thread 2 (Thread 0x7ff8d15cb700 (LWP 14006))]
> #1  0x00007ff8d2db2a75 in __GI___pthread_mutex_lock
> (mutex=0x7ff8d16ac810) at ../nptl/pthread_mutex_lock.c:80
> 80          LLL_MUTEX_LOCK (mutex);
> (gdb) f 1
> #1  0x00007ff8d2db2a75 in __GI___pthread_mutex_lock
> (mutex=0x7ff8d16ac810) at ../nptl/pthread_mutex_lock.c:80
> 80          LLL_MUTEX_LOCK (mutex);
> (gdb) p *mutex
> $2 = {__data = {__lock = 0, __count = 0, __owner = 0, __nusers = 0,
> __kind = 0, __spins = 0, __elision = 0,
>     __list = {__prev = 0x0, __next = 0x0}}, __size = '\000' <repeats
> 39 times>, __align = 0}
> 
> 
> Server back trace (All threads are just waiting around.)
> ------------------------------------------------------------------------
> Thread 6 (Thread 0x7f0686b45700 (LWP 14094)):
> #0  pthread_cond_wait@@GLIBC_2.3.2 () at
> ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
> #1  0x00007f0687838d8b in migrator (attr=0x7f0687c4f758) at hsh_int.c:207
> #2  0x00007f06875fb3e4 in start_thread (arg=0x7f0686b45700) at
> pthread_create.c:333
> #3  0x00007f0686e2d8ad in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
> 
> Thread 5 (Thread 0x7f0684b41700 (LWP 14098)):
> #0  pthread_cond_timedwait@@GLIBC_2.3.2 () at
> ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_timedwait.S:225
> #1  0x00007f0687896555 in _do_checkpoint (attr=0x0) at ckp.c:1411
> #2  0x00007f06875fb3e4 in start_thread (arg=0x7f0684b41700) at
> pthread_create.c:333
> #3  0x00007f0686e2d8ad in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
> 
> Thread 4 (Thread 0x7f0685342700 (LWP 14097)):
> #0  pthread_cond_wait@@GLIBC_2.3.2 () at
> ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
> #1  0x00007f0687838d8b in migrator (attr=0x7f0687c51280) at hsh_int.c:207
> #2  0x00007f06875fb3e4 in start_thread (arg=0x7f0685342700) at
> pthread_create.c:333
> #3  0x00007f0686e2d8ad in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
> 
> Thread 3 (Thread 0x7f0685b43700 (LWP 14096)):
> #0  pthread_cond_wait@@GLIBC_2.3.2 () at
> ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
> #1  0x00007f06878541bc in _rm_log (attr=0x0) at tlg.c:1527
> #2  0x00007f06875fb3e4 in start_thread (arg=0x7f0685b43700) at
> pthread_create.c:333
> #3  0x00007f0686e2d8ad in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
> 
> Thread 2 (Thread 0x7f0686344700 (LWP 14095)):
> #0  pthread_cond_wait@@GLIBC_2.3.2 () at
> ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
> #1  0x00007f0687853110 in flush_log_buffer (attr=0x0) at tlg.c:1385
> #2  0x00007f06875fb3e4 in start_thread (arg=0x7f0686344700) at
> pthread_create.c:333
> #3  0x00007f0686e2d8ad in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
> 
> Thread 1 (Thread 0x7f0687cef700 (LWP 14093)):
> #0  0x00007f0686e264f3 in select () at ../sysdeps/unix/syscall-template.S:84
> #1  0x00007f068785dcf0 in _daemon_server_loop (attr=0x7ffff2d255d0) at mam.c:730
> #2  0x00007f068785e1bc in mam_daemon_server_run (ctx=0x7ffff2d255d0)
> at mam.c:860
> #3  0x00007f0687870053 in dae_init (instance=0x212d180
> "/tmp/func_api", force=1, ctx=0x7ffff2d255d0)
>     at dae.c:297
> #4  0x00007f068785bf4a in _do_start_daemon (ctx=0x7ffff2d255d0) at clu.c:320
> #5  0x00007f068785c388 in clu_execute (ctx=0x7ffff2d255d0) at clu.c:449
> #6  0x0000000000400994 in main (argc=6, argv=0x7ffff2d25b58) at db.c:21
> 
> I can see that it is always server thread #5 that interferes with the
> hanging client thread in the following way:
> Server thread acquires mutex X.
> Client thread acquires mutex X (it sits waiting for the server thread
> to release mutex X).
> Server thread releases mutex X and continues its work...
> Client thread never wakes up. It continues to hang on mutex X.
> 
> I have written a test program to try to provoke the same access
> pattern but I cannot make the same thing happen.
> I can see that the same access pattern:
> 1) server acquires mutex X
> 2) client acquires mutex X
> 3) server releases mutex X
> 4) client acquires mutex X
> completes successfully numerous times before the client hangs as shown above.
> 
> Can you give me hints on how to continue further or do you want me to
> send my code to you?
> As seen above the hardware is x86_64.
> 
> Brs
> /Olle

I think either a testcase or a way to reproduce this issue would be the
way to proceed since current log is still inconclusive.  I tried to check
on sourceware bug database to check some fixed issue since 2.19, but you
seemed to already checked with a more recent one.

I strongly recommend you to try first a memory checker (valgrind, asan) 
and/or a thread checker (tsan) to see if you are doing something wrong.