Re: Bug: Incorrect signal behavior in multi-threaded processes

["E. Madison Bray" <erik.m.bray@gmail.com>]

On Tue, Jan 22, 2019 at 9:43 PM Dan Bonachea <dobonachea@lbl.gov> wrote:
>
> Hi Corinna and Madison, thanks for your responses.
>
> To clarify, I'm reasonably confident the problem I'm reporting has
> NOTHING to do with pthread_barrier. Our real application which
> exhibits very similar symptoms does not use pthread_barrier *at all*;
> pthread_barrier was merely the most convenient/concise synchronization
> mechanism to produce deterministic output behavior in the minimal
> example.
>
> Indeed, when I comment out the pthread_barrier code entirely from the
> example program (causing unselected non-primordial threads to exit and
> the primordial thread to stall in pthread_join), I see substantially
> the same misbehaviors.

Thank you for the clarification, that does make sense.

Per my previous message, the problem with SIGSEGV in particular is
(IMO a somewhat serious problem) caused by the fact that Windows
apparently does not recognize the stacks allocated for pthreads as
valid stacks, so it bypasses SEH entirely, and exceptions like
segfaults in threads do not get handled well by Cygwin.

I'm less clear on what the problem is with other signals, but it might
also be related somehow (though the SIGSEGV problem is obviously more
severe since it just results in the process immediately dying (with
exit code 0 no less :(


> On Tue, Jan 22, 2019 at 6:16 AM E. Madison Bray <erik.m.bray@gmail.com> wrote:
> >
> > On Sun, Jan 20, 2019 at 9:34 PM Dan Bonachea wrote:
> > >
> > > I'm writing to report some POSIX compliance problems with Cygwin
> > > signal handling in the presence of multiple pthreads that our group
> > > has encountered in our parallel scientific computing codes.
> > >
> > > A minimal test program is copied below and also available here:
> > > https://upc-bugs.lbl.gov/bugzilla/attachment.cgi?id=589
> > >
> > > I believe the test program is fully compliant with ISO C 99 and POSIX
> > > 1003.1-2016. In a nutshell, it registers one signal handler, spawns a
> > > number of pthreads, and then synchronously generates a signal from
> > > exactly one thread while others sit in a pthread_barrier_wait. The
> > > "throwing" thread and signal number can be varied from the command
> > > line, and diagnostic output indicates what happened.
> > >
> > > As a basis for comparison, here are a few examples of the test program
> > > running on x86_64/Linux-3.10.0(Scientific Linux 7.4)/gcc-4.8.5
> > > demonstrating what I believe to be the *correct*/POSIX-required
> > > behavior:
> >
> > Thank you for the detailed analysis of this problem.  I haven't
> > personally encountered a problem like this in any of my own code,
> > though I'm not relying on pthread_barrier, or signal handlers being
> > run from specific threads.  This is relevant to my interests though,
> > so time permitting I might look into it just out of curiosity of
> > nothing else.  The behavior with SIGSEGV in particular is very
> > reminiscent (possibly same as) a problem I reported last year, but
> > never got around to fixing (except for the local workaround I used):
> > https://cygwin.com/ml/cygwin/2018-05/msg00333.html
> >
> > I wonder if the same problem applies to thread-local stacks.  Indeed,
> > I ran your test program in gdb with arguments (1, 11) with a
> > breakpoint in myfault_altstack_handler [1] and wound up there.  But
> > since the segfault did not come from Cygwin itself (me.andreas is a
> > "san" fault handler for the current exception being handled by Cygwin,
> > but this is only set for exceptions generated by Cygwin itself (with
> > its __try/__except blocks).  In this case it's 0x0 so the exception is
> > not handled and the process just runs off into the weeds until it
> > (quickly) runs out of "vectored continue handlers" and so the process
> > exits (at the Windows level, without Cygwin controlling its shutdown).
> >
> > For the other cases I'm not as sure what's going on, but possibly
> > related problems.
> >
> > [1] https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/exceptions.cc;h=205ad850e4c7b69954fadd1efe3ae9ff65e5f806;hb=HEAD#l594

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