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Date: Tue, 1 Nov 2011 11:28:11 -0400
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From: Ryan Stone <rysto32@gmail.com>
To: Attilio Rao <attilio@freebsd.org>
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Cc: mdf@freebsd.org, FreeBSD Current <freebsd-current@freebsd.org>,
	mlaier@freebsd.org
Subject: Re: smp_rendezvous runs with interrupts and preemption enabled on
 unicore systems
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On Mon, Oct 31, 2011 at 7:43 PM, Attilio Rao <attilio@freebsd.org> wrote:
> I'm not entirely sure why this exactly breaks though (do you see that
> happening with a random rendezvous callback or it is always the
> same?), because that just becames a simple function calling on cpu0,
> even if I think that there is still a bug as smp_rendezvous callbacks
> may expect to have interrupts and preemption disabled and the
> short-circuit breaks that assumption.

I have only observed the problem with i686_mrstoreone(smp_rendezvous
is called from i686_mrstore).

I notice that i686_mrstoreone does scary things like disabling the
cache and global TLB entries.  My experience was that everything got
very slow when i686_mrstoreone was preempted, so it easily could be
the case that the system because so slow with the cache disabled that
it became livelocked.

> I don't think that we strictly need the lock here, my preferred
> solution would be (only test-compiled):

I tried the same thing for the SMP case, and it fixed my problem.