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

Corinna Vinschen corinna-cygwin@cygwin.com
Mon Jul 17 11:05:08 GMT 2023


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



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