This is the mail archive of the
cygwin
mailing list for the Cygwin project.
Re: Possible race in SYSV IPC (semaphores)
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
--
Problem reports: http://cygwin.com/problems.html
FAQ: http://cygwin.com/faq/
Documentation: http://cygwin.com/docs.html
Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple