c++ - pthread_mutex_lock blocks but __lock = 0 -

i writing multi-threaded program, , running deadlock.

one of threads blocks while other threads sleeping (cond_wait)

so entered ctrl+c go gdb terminal

(gdb) info thread   5 thread 0x1c6ff4a0 (lwp 723)  __pthread_cond_wait (cond=0x420c50, mutex=0x420c34) @ pthread_cond_wait.c:156   3 thread 0x1bcf34a0 (lwp 721)  __pthread_cond_wait (cond=0x41a530, mutex=0x41a514) @ pthread_cond_wait.c:156 * 1 thread 0x1b2c9720 (lwp 716)  __lll_lock_wait (futex=0x1be08240, private=0) @ ../nptl/sysdeps/unix/sysv/linux/lowlevellock.c:46 (gdb) bt #0  __lll_lock_wait (futex=0x1be08240, private=0) @ ../nptl/sysdeps/unix/sysv/linux/lowlevellock.c:46 #1  0x1afa1540 in __pthread_mutex_lock (mutex=0x1be08240) @ pthread_mutex_lock.c:61 #2  0x1abb7984 in readerwait (frameinfo=0x1be08240, sem=0x1aad4000) @ camipcsource.cpp:282 ...... 

i can see thread blocked @ camipcsource.cpp:282

but when change frame #2 , print mutex

(gdb) frame 2 #2  0x1abb7984 in readerwait (frameinfo=0x1be08240, sem=0x1aad4000) @ camipcsource.cpp:282 (gdb) p frameinfo->m_reader $1 = {__data = {__lock = 0, __count = 0, __owner = 0, __kind = 0, __nusers = 0, {   __spins = 0, __list = {__next = 0x0}}}, __size = '\000' <repeats 23 times>, __align = 0} 

isn't weird? block on nonblock mutex?

and when let gdb continue, program lock

my question: else can know reason why thread block @ non-block mutex?

environment: compiled mipsel-linux-gnu-g++-4.4, running on mips (libpthread-2.11.3)

here code snippet(camipcsource.cpp) thread blocks:

277 int readerwait(frame_info_t* frameinfo, sem_t* sem) { 278     int ret; 279 280     if (frameinfo == null || sem == null)   return -1; 281 282     pthread_mutex_lock(&frameinfo->m_reader); 283     /*while ((ret = pthread_mutex_trylock(&frameinfo->m_reader)) != 0) { 284         if (ret == ebusy) 285             fprintf(stderr, "-"); 286         else { 287             _err("pthread_mutex_trylock:(%s)\n", strerror(ret)); 288             return -1; 289         }                                                                                               290     }*/ 291                                                                                            292     frameinfo->readercount++; 293     if (frameinfo->readercount == 1) { 294         if (sem_wait(sem) != 0) { 295             _err("sem_wait:(%s)\n", strerror(errno)); 296             frameinfo->readercount--; 297             if ((ret = pthread_mutex_unlock(&frameinfo->m_reader)) != 0); 298                 _err("pthread_mutex_unlock:(%s)\n", strerror(ret)); 299             return -1; 300         } 301      } 302     if ((ret = pthread_mutex_unlock(&frameinfo->m_reader)) != 0) 303         _err("pthread_mutex_unlock:(%s)\n", strerror(ret)); 304 305     return 0; 306 } 

btw, i've tried replace pthread_mutex_lock pthread_mutex_trylock @ line 283~290

and works quite "-" output :(

here readersignal function (readerwait/readersignal 2 function access mutex)

339 int readersignal(frame_info_t* frameinfo, sem_t* sem) { 340     int ret; 341 342     if (frameinfo == null || sem == null)   return -1;  343 344     pthread_mutex_lock(&frameinfo->m_reader); 345     /*while ((ret = pthread_mutex_trylock(&frameinfo->m_reader)) != 0) { 346         if (ret == ebusy) 347             fprintf(stderr, "-"); 348         else { 349             _err("pthread_mutex_trylock:(%s)\n", strerror(ret)); 350             return -1; 351         } 352     }*/ 353     frameinfo->readercount--; 354     if (frameinfo->readercount == 0) { 355         if (sem_post(sem) != 0) { 356             _err("sem_post:(%s)\n", strerror(errno)); 357             frameinfo->readercount++; 358             if ((ret = pthread_mutex_unlock(&frameinfo->m_reader)) != 0) 359                 _err("pthread_mutex_unlock:(%s)\n", strerror(ret)); 360             return -1; 361         } 362 363     } 364     if ((ret = pthread_mutex_unlock(&frameinfo->m_reader)) != 0) 365         _err("pthread_mutex_unlock:(%s)\n", strerror(ret)); 366 367     return 0; 368 } 

debugging multithreaded programs gdb typically difficult. better use tool designed handle multithreaded issues. highly recommend looking @ helgrind:

http://valgrind.org/docs/manual/hg-manual.html

from page: "helgrind valgrind tool detecting synchronisation errors in c, c++ , fortran programs use posix pthreads threading primitives."

it supply information on potential race conditions, deadlock, etc. pretty awesome really. has saved me more once.

good luck!