STC for libapr1 failure

Corinna Vinschen
Sat Aug 27 20:37:00 GMT 2011

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 Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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