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Date: Fri, 24 Oct 2014 15:52:31 +0200
From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: Threads
Message-ID: <20141024135231.GL20607@calimero.vinschen.de>
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References: <54450835.3050602@cornell.edu> <5448E6F9.8040005@dronecode.org.uk> <5448EEBF.3020706@cornell.edu> <20141023153730.GC20607@calimero.vinschen.de> <544A327E.9090006@dronecode.org.uk> <20141024125416.GK20607@calimero.vinschen.de>
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On Oct 24 14:54, Corinna Vinschen wrote:
> On Oct 24 12:05, Jon TURNEY wrote:
> > On 23/10/2014 16:37, Corinna Vinschen wrote:
> > >On Oct 23 08:04, Ken Brown wrote:
> > >>Yes, flags register corruption is exactly what Eli suggested in the o=
ther
> > >>bug report I cited.
> > >
> > >The aforementioned patch was supposed to fix this problem and it is
> > >definitely in the current 1.7.32 release...
> >=20
> > I didn't mean to suggest otherwise, just that perhaps a similar problem
> > exists now.
> >=20
> > So I made the attached test case to explore that.  Maybe I've made an
> > obvious mistake with it, but on the face of it, it seems to demonstrate
> > something...
> >=20
> > jon@tambora /
> > $ gcc signal-stress.c  -Wall -O0 -g
> >=20
> > jon@tambora /
> > $ ./a
> > failed: 2144210386 isn't equal to 2144210386, apparently
>=20
> So it checks i and j for equality, fails, and then comes up with
> "42 isn't equal to 42"?  This is weird...
>=20
> > Note there is some odd load dependency. For me, it works fine when it's=
 the
> > only thing running, but when I start up something CPU intensive, it oft=
en
> > fails...
>=20
> That's... interesting.  I wonder if that only occurs in multi-core or
> multi-CPU environments.  The fact that i and j are not the same when
> testing, but then are the same when printf is called looks like a
> out-of-order execution problem.
>=20
> Is it possible that we have to add CPU memory barriers to the sigdelayed
> function to avoid stuff like this?

I discussed this with my college Kai Tietz (many thanks to him from
here), and we came up with a problem in sigdelayed in the 64 bit case:
pushf is called *after* aligning the stack with andq.  This alignment
potentially changes the CPU flag values so the restored flags are
potentially not the flags when entering sigdelayed.

I just applied a patch and created new snapshots on
https://cygwin.com/snapshots/

I couldn't reprocude the problem locally, so I'd be grateful if you
could test if that fixes the problem in your testcase, Jon.

Ken, can you check if this snapshot helps emacs along, too?


Thanks,
Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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