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Re: shared data protection failed in pthread_cond_timedwait


On 2014/4/25 17:43, Will Newton wrote:
> On 25 April 2014 07:58, Yang Yingliang <yangyingliang@huawei.com> wrote:
>> Hi,
>>
>>
>> I have 22 threads wait in pthread_cond_timedwait. When they are all woke up, I found
>> there are more than one threads can access shared data in pthread_cond_timedwait.
>>
>> I added print messages as follow code:
>>
>> --- libc/nptl/pthread_cond_timedwait.c
>> +++ libc/nptl/pthread_cond_timedwait.c
>> @@ -34,6 +34,7 @@
>>  #else
>>  # include <bits/libc-vdso.h>
>>  #endif
>> +#include <stdio.h>
>>
>>  /* Cleanup handler, defined in pthread_cond_wait.c.  */
>>  extern void __condvar_cleanup (void *arg)
>> @@ -235,7 +239,9 @@
>>
>>   bc_out:
>>
>> +printf("start do sub :%d, lock:%d %p\n", cond->__data.__nwaiters, cond->__data.__lock, pthread_self());
>>    cond->__data.__nwaiters -= 1 << COND_NWAITERS_SHIFT;
>> +printf("end do sub :%d, lock:%d %p\n", cond->__data.__nwaiters, cond->__data.__lock, pthread_self());
>>
>>    /* If pthread_cond_destroy was called on this variable already,
>>       notify the pthread_cond_destroy caller all waiters have left
>>
>>
>> I tested on Linux arma15el 3.10.37+ #2 SMP Fri Apr 25 11:23:25 CST 2014 armv7l GNU/Linux.
>> Here is the result:
>>
>> start do sub :45, lock:1 0xb6d9a460
>> end do sub :43, lock:1 0xb6d9a460
>> start do sub :43, lock:1 0xb6d9e460
>> end do sub :41, lock:2 0xb6d9e460
>> start do sub :43, lock:2 0xb6dbe460    //two threads both access the shared data
>> start do sub :41, lock:1 0xb6daa460
>> end do sub :39, lock:2 0xb6daa460
>> start do sub :39, lock:2 0xb6de6460
>> end do sub :37, lock:2 0xb6de6460
>> start do sub :37, lock:2 0xb6db6460
>> end do sub :35, lock:2 0xb6db6460
>> start do sub :35, lock:2 0xb6dc2460
>> end do sub :33, lock:2 0xb6dc2460
>> end do sub :37, lock:2 0xb6dbe460
>> start do sub :33, lock:2 0xb6dc6460
>> end do sub :31, lock:0 0xb6dc6460
>> start do sub :31, lock:2 0xb6dae460
>> end do sub :29, lock:2 0xb6dae460
>> start do sub :29, lock:2 0xb6db2460
>> end do sub :27, lock:2 0xb6db2460
>> start do sub :27, lock:2 0xb6dba460
>> end do sub :25, lock:2 0xb6dba460
>> start do sub :25, lock:2 0xb6da2460
>> end do sub :23, lock:2 0xb6da2460
>>
>> Is lll_lock (cond->__data.__lock, pshared) failed?
>>
>> pshared is LLL_SHARED.
> 
> I have had a quick look at this and there is no obvious reason I can
> see for this behaviour, unless there is some way that IO buffering
> could cause the messages to be strangely interleaved. The other
> alternative that may be worth investigating is whether or not
> ldrex/strex is working correctly in your SMP system.
> 

After doing some investigation, it looks like atomic_compare_and_exchange_val_acq
not doing atomic. So two threads can both acquire lock when futex is 0. Is there
something wrong in atomic_compare_and_exchange_val_acq ?

#define __lll_lock(futex, private)					      \
  ((void) ({								      \
    int *__futex = (futex);						      \
    if (__builtin_expect (atomic_compare_and_exchange_val_acq (__futex,       \
								1, 0), 0))    \
      {									      \
	if (__builtin_constant_p (private) && (private) == LLL_PRIVATE)	      \
	  __lll_lock_wait_private (__futex);				      \
	else								      \
	  __lll_lock_wait (__futex, private);				      \
      }									      \
  }))


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