Date: Sun, 15 May 2011 10:12:48 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: John Baldwin <jhb@FreeBSD.org> Cc: FreeBSD current <freebsd-current@FreeBSD.org>, Peter Grehan <grehan@FreeBSD.org> Subject: Re: proposed smp_rendezvous change Message-ID: <4DCF7CF0.1080508@FreeBSD.org> In-Reply-To: <4DCE9EF0.3050803@FreeBSD.org> References: <4DCD357D.6000109@FreeBSD.org> <4DCE9EF0.3050803@FreeBSD.org>
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on 14/05/2011 18:25 John Baldwin said the following: > Hmmm, so this is not actually sufficient. NetApp ran into a very similar race > with virtual CPUs in BHyVe. In their case because virtual CPUs are threads that > can be preempted, they have a chance at a longer race. > > The problem that they see is that even though the values have been updated, the > next CPU to start a rendezvous can clear smp_rv_waiters[2] to zero before one of > the other CPUs notices that it has finished. As a follow up to my previous question. Have you noticed that in my patch no slave CPU actually waits/spins on smp_rv_waiters[2]? It's always only master CPU (and under smp_ipi_mtx). -- Andriy Gapon
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