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Date: Mon, 6 Aug 2012 10:58:17 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Cc: Andrey Khalyavin <halyavin AT google DOT com>
Subject: Re: Race condition that leads to random crashes in cygwin-based builds.
Message-ID: <20120806085817.GA21876@calimero.vinschen.de>
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References: <CACgPhYpV-M477Mfugp3DjdWCEvbJ-8LhyH12y=iGP_MuK6VTAQ AT mail DOT gmail DOT com> <20120724135718 DOT GA29107 AT calimero DOT vinschen DOT de>
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Andrey?

On Jul 24 15:57, Corinna Vinschen wrote:
> On Jul 24 17:25, Andrey Khalyavin wrote:
> > Hi, we have build bots that crash randomly on Windows XP and rarely on
> > Windows 7.
> > [...]
> > Investigation of this crash dump showed that wincapc::init in
> > winsup\cygwin\wincap.cc
> > called api_fatal ("Cygwin requires at least Windows 2000."). This
> > function is called at
> > cygwin1.dll initialization even before any code in our compiler
> > (cc1.exe) have been
> > executed. Further investigation showed that wincapc variable is in
> > shared section:
> > wincapc wincap __attribute__((section (".cygwin_dll_common"), shared));
> > but wincapc::init() function doesn't have any synchronization and is called from
> > dll_crt0_0 without any synchronization. Using shared variables without
> > synchronization
> > is sure way to get random failures. Here is one scenario that can lead
> > to api_fatal called:
> > [...]
> > 3. First process enters wincapc::init, clears version field with
> > memset and executes
> > version.dwOSVersionInfoSize = sizeof (OSVERSIONINFOEX)
> > 4. Task switching happens and second process enters wincapc::init. It
> > sees that caps
> > field is still not initialized yet and cleaders version field with memset.
> > 5. Task switching happens and first process proceeds to execute
> > GetVersionEx with
> > version cleared by memset and so not having its size set.
> > 6. GetVersionEx returns error and first process fails to start.
> > 
> > If there is no easy way to add synchronization to wincapc::init, I
> > suggest to make
> > wincap a regular (not shared) variable.
> 
> There's another way, afaics.  The idea here was that wincap is only
> ever set once, and even *if* the information is written twice, the
> content will be identical.
> 
> So, afaics, the above problem is a result of using memset at all.  At
> startup, wincap is all 0 anyway, so the memset is not required and
> apparently it even hurts.  Weird that nobody saw this problem before.
> 
> I applied a patch which should fix this problem.  Please give the
> next developer snapshot from http://cygwin.com/snapshots/ a try,
> or build yourself from CVS.

Ping?  Any feedback?  Did you ever try a snapshot?


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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