Re: Possible race in SYSV IPC (semaphores)

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

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...


Corinna

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

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