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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] Re: [PATCH] Fix freezing bug in curses console
Date: Sun, 1 Mar 2009 13:03:37 +0000
References: <gl7g9p$jpn$2@ger.gmane.org> <20090228212116.GL20640@shareable.org>
	<20090301113635.GA10538@redhat.com>
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To: qemu-devel@nongnu.org, "Daniel P. Berrange" <berrange@redhat.com>
Cc: Matthew Bloch <matthew@bytemark.co.uk>, kvm@vger.kernel.org

> > > I think it still suffers from the same race condition so today it
> > > wouldn't work.  You could fix the bottom half scheduling though so that
> > > you could safely schedule a bottom half from a signal handler (using
> > > roughly the same trick).
> >
> > Fwiw, it's perfectly sensible to have a single pipe which is shared by
> > all signal handlers, just used to say "check for work flags set".
>
> And if you need the main loop to be able to distinguish signals coming
> out of the pipe, then just write the signum into the pipe as a byte,
> instead of a single dummy byte. Or even write the whole 'siginfo_t'
> struct passed to the signal handler, and read it out in sizeof(siginfo_t)
> sized chunks for processing.

I don't think this will works. If the pipe buffer gets full the write will 
either block or you'll loose signals.

When using the pipe as a simple semaphore all you care about is the presence 
or absence of data. It doesn't matter if subsequent writes loose data (e.g. 
by not retrying a nonblocking write) as long as a write to an empty pipe 
succeeds.

Paul