Subj : Re: Win32 condition variables redux
To   : comp.programming.threads
From : Chris Thomasson
Date : Tue Aug 30 2005 06:55 pm

<Charles Bryant> wrote:

> skip down to:



>>void condvar_broadcast( condvar_t *_this )
...
>>    LeaveCriticalSection( &_this->mutex );

>>    while ( thread ) { next = thread->next;



>Note that at this point we don't hold any mutex

This iteration is completly private. More on this below...




>while in:


>>void prv_condvar_pop( condvar_t *_this, per_thread_t *thread ) {
...
>> thread->next = 0; thread->prev = 0; }


>called from


>>int prv_condvar_timeout( condvar_t *_this, per_thread_t *thread ) { int 
>>err = 0, wait = 0; LONG count = 0; EnterCriticalSection( &_this->mutex ); 
>>if ( thread->state ) { prv_condvar_pop( _this, thread );


> So if a thread gets a timeout just as a condvar_broadcast() is running 
> through the list of threads, then the thread may corrupt the list of 
> threads to wake, leading to threads never being woken.

> To test, use this scenario:


> Modify the code to insert 'Sleep(10 seconds)' just after the 
> LeaveCriticalSection in condvar_broadcast()


> Run the following: Start N threads which do condvar_timedwait() with no 
> timeout. Sleep one second to ensure that they have gone asleep Start a 
> thread which does condvar_timedwait() with a timeout
                of 4 seconds
>        wait 1 second Start N threads which do condvar_timedwait() with no 
> timeout. wait 1 second perform condvar_broadcast() wait 15 seconds


>        check how many threads were woken. It should be 2N+1, but I believe 
> it will be N+1


The race-condition you describe can't happen because the broadcast function 
pops the entire list of nodes, sets their per_thread_t::state variables to 
0, and stores the front pointer locally under the protection of the mutex. 
Notice how the per_thread_t::state variable is always protected by the mutex 
and is used to indicate whether or not a thread is in the list? How can the 
timeout function corrupt the list when it only pops threads that have their 
per_thread_t::state variables set to PER_THREAD_WAITING? I think you need to 
study the pseudo-code a some more...




Keep this in mind:

/* not in list */
per_thread_t::state == 0;


/* in list */
per_thread_t::state == PER_THREAD_WAITING;


;)

.