Re: pthread_cond_wait

[Polina Dudnik <pdudnik at cs dot wisc dot edu>]

Hi,

Thank you for the reply. I have made an attempt at implementing a 
mutex-less pthread_cond_wait and pthread_cond_signal.

Specifically I am using futexes to implement it (below).

I define a global integer initialized to zero, which is my futex.

So, my cond_wait function looks like this:

int rn = lll_futex_wait(&event, event);

cond_wait looks like this:

GetLock(&lock, threadid+1);
event++;
ReleaseLock(&lock);
lll_futex_wake(&event, INT_MAX);

And cond_broadcast looks like this:

GetLock(&lock, threadid+1);
event++;
ReleaseLock(&lock);
lll_futex_wake(&event, 1);


However, I am still loosing signals and I don't know why. If anybody is 
strong with futexes, I would really appreciate any information. Thanks.





#define lll_futex_wait(futex, val) \
  ({									      \
    register const struct timespec *__to __asm ("r10") = NULL;	 
      \
    int __status;							      \
    register __typeof (val) _val __asm ("edx") = (val);			      \
    __asm __volatile ("syscall"						      \
		      : "=a" (__status)					      \
		      : "0" (SYS_futex), "D" (futex),			      \
			"S" (FUTEX_WAIT),	 		  	      \
			"d" (_val), "r" (__to)				      \
		      : "memory", "cc", "r11", "cx");			      \
    __status;								      \
  })


#define lll_futex_wake(futex, nr) \
  do {									      \
    int __ignore;							      \
    register __typeof (nr) _nr __asm ("edx") = (nr);			      \
    __asm __volatile ("syscall"						      \
		      : "=a" (__ignore)					      \
		      : "0" (SYS_futex), "D" (futex),			      \
			"S" (FUTEX_WAKE),  	                              \
			"d" (_nr)					      \
		      : "memory", "cc", "r10", "r11", "cx");		      \
  } while (0)



Gilles Carry wrote:
> Polina Dudnik wrote:
> > Hi,
> >
> > I would like to write my own version of pthread_cond_wait, which 
> > doesn't have a mutex as one of it's parameters, to be used with 
> > transactional memory model. I am not sure why it is important to have 
> > the mutex locked before calling lll_lock (cond->__data.__lock, 
> > pshared). What would happen if the mutex was unlocked before calling 
> > lll_lock (cond->__data.__lock, pshared). Thanks.
>
> Hi,
> As far as I remember, the external mutex is to avoid loss of signals.
> Releasing the external mutex before the internal one would let a time 
> window
> in which a signal could be issued before the waiter is put into the 
> waiting queue.
> Anyway, before modifying pthread_cond_* stuff, you should carefully read 
> this:
> http://www.opengroup.org/onlinepubs/009695399/functions/pthread_cond_wait.html 
>
> and especially "Features of Mutexes and Condition Variables".
>
> Hope this helps.
>
> Gilles.