Am Mittwoch, 5. Dezember 2007 schrieb Julian Seward:
> On Wednesday 05 December 2007 16:32, Christoph Bartoschek wrote:
> > The read of COND in the parent thread happens in a segment that is after
> > the accesses that established the shared-modified state in the
> > happens-before graph.
> >
> > Given that COND should not trigger an error.
>
> But why should we assume that ownership of COND should change from
> shared-modified to exclusively-owned-by-parent at the point of the
> signal/wait pair?
Currently I think that one does not change the state of the variable. But when
one detects an error with the current approach, one would check the
happens-before graph to see whether there really is a problem.
> In this example, it would cause Helgrind to shut up, but it's not obvious
> to me that this is a good thing to do in general.
>
> It would be easy enough (although expensive) to modify Helgrind so that,
> when thread X signals at Y, memory shared by X and Y is made exclusive
> to Y. Is that helpful or correct? I don't know. I don't know what the
> consequences would be, just now.
There is one problem when several signals are emitted. Which one should one
use to establish the happens-before relation? This could make the tool
dependent on the scheduler. For example the following code:
Thread 1: Thread 2:
a. pthread_mutex_lock(mutex); 1. pthread_mutex_lock(mutex);
b. while (COND != 1) { 2. COND = 1;
c. pthread_cond_wait(cv); 3. pthread_cond_signal(cv);
d. } 4. pthread_mutex_unlock(mutex);
e. pthread_mutex_unlock(mutex); 5. pthread_mutex_lock(mutex);
f. COND = 3; 6. COND = 2;
7. pthread_cond_signal(cv);
8. pthread_mutex_unlock(mutex);
If thread 1 wakes up on the first signal in line 3, then there is a problem in
lines f and 6. If thread 1 wakes up on the second signal in line 7, one would
not detect an error. Detecting the error in the first case would be the hard
case in my opinion.
...
> Instead of using source annotations, you can also avoid this
> missing-dependency problem with CVs by using semaphores instead.
> Helgrind tracks dependencies through semaphores exactly. A semaphore
> with an initial value of zero is useful for signalling events between
> threads, without having to mess with these boolean conditions on
> CVs.
You are right. But lots of code uses condition variables. I would like to use
helgrind on several code parts that use condition variables, but currently
the false positive count is too high.
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