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

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

On Jul 17 12:51, Jon Turney wrote:
> On 14/07/2023 14:04, Jon Turney wrote:
> > On 13/07/2023 19:53, Corinna Vinschen wrote:
> > > > > > normally after 10 seconds. (See the commentary in pthread::cancel() in
> > > > > > thread.cc, where it checks if the target thread is inside the kernel,
> > > > > > and silently converts the cancellation into a deferred one)
> > > > > 
> > > > > 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.
> > 
> > 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.
> > 
> > I guess this could also check that not all of the threads ran for all 10
> > seconds, which would indicate that at least some of them were cancelled
> > asynchronously.
> 
> I wrote this, attached, which does indeed make this test more reliable.
> 
> You point is well made that this is making assumptions about how quickly
> async cancellation works that are not required by the standard
> 
> (It would be a valid, if strange implementation, if async cancellation
> always took 10 seconds to take effect, which this test assumes isn't the
> case)
> 
> Perhaps there is a better way to write a test that async cancellation works
> in the absence of cancellation points, but it eludes me...

Same here, so just go ahead.

Thanks,
Corinna