Re: [PATCH 08/11] Cygwin: testsuite: Busy-wait in cancel3 and cancel5

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

On Jul 14 14:04, Jon Turney wrote:
> On 13/07/2023 19:53, Corinna Vinschen wrote:
> > > > Nevertheless, I think this is ok to do.  The description of pthread_cancel
> > > > contains this:
> > > > 
> > > >    Asynchronous cancelability means that the thread can be canceled at
> > > >    any time (usually immediately, but the system does not guarantee this).
> > > > 
> > > > And
> > > > 
> > > >    The above steps happen asynchronously with respect to the
> > > >    pthread_cancel() call; the return status of pthread_cancel() merely
> > > >    informs the caller whether the cancellation request was successfully
> > > >    queued.
> > > > 
> > > > So any assumption *when* the cancallation takes place is may be wrong.
> 
> Yeah.
> 
> I think the flakiness is when we happen to try to async cancel while in the
> Windows kernel, which implicitly converts to a deferred cancellation, but
> there are no cancellation points in the the thread, so it arrives at
> pthread_exit() and returns a exit code other than PTHREAD_CANCELED.

In pthread_join(), right?

> I did consider making the test non-flaky by adding a final call to
> pthread_testcancel(), to notice any failed async cancellation which has been
> converted to a deferred one.
> 
> But then that is just the same as the deferred cancellation tests, and
> confirms the cancellation happens, but not that it's async, which is part of
> the point of the test.

What if Cygwin checks for a deferred cancellation in pthread::exit,
too?  It needs to do this by its own, not calling pthread::testcancel,
otherwise we're in an infinite loop.  Since cancel is basically like
exit, just with a PTHREAD_CANCELED return value, the only additional
action would be to set retval to PTHREAD_CANCELED explicitely.


Corinna