On Jul 14 20:57, Corinna Vinschen wrote:
> 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.
Kind of like this:
diff --git a/winsup/cygwin/thread.cc b/winsup/cygwin/thread.cc
index f614e01c42f6..fceb9bda1806 100644
--- a/winsup/cygwin/thread.cc
+++ b/winsup/cygwin/thread.cc
@@ -546,6 +546,13 @@ pthread::exit (void *value_ptr)
class pthread *thread = this;
_cygtls *tls = cygtls; /* Save cygtls before deleting this. */
+ /* Deferred cancellation still pending? */
+ if (canceled)
+ {
+ WaitForSingleObject (cancel_event, INFINITE);
+ value_ptr = PTHREAD_CANCELED;
+ }
+
// run cleanup handlers
pop_all_cleanup_handlers ();
What do you think?
Corinna
