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<b><a class="bz_bug_link
bz_status_NEW "
title="NEW - Concurrent call to glClientWaitSync results in segfault in one of the waiters."
href="https://bugs.freedesktop.org/show_bug.cgi?id=98172#c11">Comment # 11</a>
on <a class="bz_bug_link
bz_status_NEW "
title="NEW - Concurrent call to glClientWaitSync results in segfault in one of the waiters."
href="https://bugs.freedesktop.org/show_bug.cgi?id=98172">bug 98172</a>
from <span class="vcard"><a class="email" href="mailto:shinji.suzuki@gmail.com" title="shinji.suzuki@gmail.com">shinji.suzuki@gmail.com</a>
</span></b>
<pre>Sorry I've made too many mistake in writing. (Should have gone to bed before
writing.) I'll rewrite whole post below.
I'm afraid execution of st_fence_sync() can still race.
Thread-A can run upto
success = screen->fence_finish(screen, pipe, fence, 0);
and then get preempted and Thread-B can run upto the same location.
And then
screen->fence_reference(screen, &so->fence, NULL);
will be executed serially by both threads. If screen->fence_finish()
is thread-safe and return true only for the first invocation then all is fine
but likely it is not true as otherwise we will not be struggling with this
issue.
I think the gist of it is that checking of so->fence and nullifying of it
should be executed atomically. If "if (success)" is replaced with "if (success
&& so->fence)" then the program may behave correctly but I'm still not
comfortable about sreen->fence_finish() being called concurrently.
I'm also concerned that mutual exclusion on ctx->Shared->Mutex may introduce
unnecessarily strict serialization.
Can't we introduce per sync-object mutex so that excution of checking of
so->fence and nullyfying of it happen atomically?
Is that modification too intrusive? (At least it is unnecessary overhead when
st_fence_sync() is not executed concurrently on the same sync object.)</pre>
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