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Date: Fri, 23 Nov 2012 17:44:04 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Possible race in SYSV IPC (semaphores)
Message-ID: <20121123164404.GX17347@calimero.vinschen.de>
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On Nov 23 14:33, Corinna Vinschen wrote:
> On Nov 23 14:10, Corinna Vinschen wrote:
> > On Nov 23 12:36, Corinna Vinschen wrote:
> > > On Nov 19 21:06, Lavrentiev, Anton (NIH/NLM/NCBI) [C] wrote:
> > > > Hello again,
> > > > 
> > > > I can now positively confirm the race condition in cygserver w.r.t. the named
> > > > pipe used to serialize SYSV requests through the server.  The race is due to
> > > > that transport_layer_pipes::accept() (bool *const recoverable) (file: transport_pipes.cc) does actually _create_ the pipe when pipe_instance == 0
> > > > (ironically, transport_layer_pipes::listen() does not create any OS primitives
> > > > at all!).
> > > > 
> > > > This means that under heavy load, cygserver threads may all end up processing
> > > > their requests and closing all instances of the pipe (bringing pipe_instance == 0)
> > > > yet not being able to get to the point of accepting new request (that is, to
> > > > re-create the pipe).  For the client (user process), this looks like the pipe
> > > > does not exist (during that very tiny period of time), and the following message
> > > > gets printed:
> > > > 
> > > > Iteration 3016
> > > >       1 [main] a 4872 transport_layer_pipes::connect: lost connection to cygserver, error = 2
> > > 
> > > Thanks for analyzing this situation.  IIUC, that means if we create the
> > > pipe in listen(), and then create another pipe instance per accepting
> > > thread in accept(), we will always have at least one instance of the pipe,
> > > whatever the load, right?
> > > 
> > > Are you set up to test a patch?  If so, I'd propose the below patch.  It
> > > would be nice if you could test it in your environment.
> > 
> > Forget it.  That doesn't work.
> > 
> > Surprisingly, the next client calling CreateFile is *not* connected to
> > any server side of the pipe which calls ConnectNamedPipe, but apparently
> > the pipes are used in the order of creation.  Thus, the first client
> > hangs waiting for the pipe instance created in the listen() method, but
> > since the server never calls ConnectNamedPipe on that instance, the
> > client will wait forever.
> > 
> > Bummer.
> > 
> > Back to the drawing board...
> 
> Try the below one.  It also connects to the listening pipe in listen(),
> so there's a single instance of the pipe connected and blocked for
> further use forever.  This should avoid the race (*and* work...)
> Please give it a try.

You can also simply try the today's snapshot version of cygserver from
http://cygwin.com/snapshots/  It contains this patch, as well as the
semaphore patch I mailed to the other thread.


Corinna

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

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