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Re: STC for libapr1 failure
On Aug 26 13:15, Corinna Vinschen wrote:
> On Aug 25 17:39, David Rothenberger wrote:
> > For a while now, the test cases that come with libapr1 have been
> > bombing with this message:
> >
> > *** fatal error - NtCreateEvent(lock): 0xC0000035
> >
> > I finally took some time to investigate and have extracted a STC
> > that demonstrates the problem.
>
> Thanks a lot for the testcase. In theory, the NtCreateEvent call should
> not have happened at all, since it's called under lock, and the code
> around that should have made sure that the object doesn't exist at the
> time.
>
> After a few hours of extrem puzzlement, I now finally know what happens.
> It's kinda hard to explain.
>
> A lock on a file is represented by an event object. Process A holds the
> lock corresponding with event a. Process B tries to lock, but the lock
> of process A blocks that. So B now waits for event a, until it gets
> signalled. Now A unlocks, thus signalling event a and closing the handle
> afterwards. But A's time slice isn't up yet, so it tries again to lock
> the file, before B returned from the wait for a. And here a wrong
> condition fails to recognize the situation. It finds the event object,
> but since it's recognized as "that's me", it doesn't treat the event as
> a blocking factor. This in turn is the allowance to create its own lock
> event object. However, the object still exists, since b has still an
> open handle to it. So creating the event fails, and rightfully so.
>
> What I don't have is an idea how to fix this problem correctly. I have
> to think about that. Stay tuned.
Please test the latest snapshot. It should fix this problem, as well as
a starvation problem with signals (and, fwiw, thread cancel events) in
flock, lockf, and POSIX fcntl locks.
Thanks again for the testcase. It was very helpful to test both problems.
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
Red Hat
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