Re: Race condition in list_consume()

Hi Andreas,

I don't see any problem in gwlist/log. pthread_cond_wait releases the mutex
passed to it. Seems you have race on some other place. 

Andreas Fink wrote:

> We found a severe bug in gwlib.
> 
> We have the following scenario:
> 
> A  calls debug("xxx",0,"xxxx") which does :
> 
>                  gwlist_add_producer(writers);
> 
> and continues but doesnt reach yet this line:
> 
>                  gwlist_remove_producer(writers);
> 
> 
> at this point the list "writers" is empty but has writers-
>  >num_producers=1
> 
> B does:
> lock(list); /* atomic lock */
> 
>          list->single_operation_lock->owner = -1;
>          pthread_cond_wait(&list->nonempty, &list-
>  >single_operation_lock->mutex);
> 
> so it waits until A is calling gwlist_remove_producer()
> 
> and wait until A completes.
> 
> 
> Now A is calling this:
> 
> void gwlist_remove_producer(List *list)
> {
>      lock(list);
>      gw_assert(list->num_producers > 0);
>      --list->num_producers;
>      pthread_cond_broadcast(&list->nonempty);
>      unlock(list);
> }
> 
> and gets locked up because the list's atomic lock is locked by B.
> 
> 
> C now has a new debug message and gets stopped at  gwlist_produce().
> 
> 
> In other words, every process who wants to write to debug log gets
> stuck.
> 
> Now there is different solutions to this.
> Our approach would be to do in gwlist_consume() to do this:
> 
> 
> unlock(list);
>          pthread_cond_wait(&list->nonempty, &list-
>  >single_operation_lock->mutex);
>          lock(list);
> 
> 
> Any other ideas?
> maybe no atomic lock around gwlist_remove_producer() ?
> 
> 
> Andreas Fink
> 
> Fink Consulting GmbH
> Global Networks Schweiz AG
> BebbiCell AG
> 
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-- 
Thanks,
Alex