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Date: Thu, 15 Nov 2018 18:50:13 -0500
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20181115235013.GA17566@flamenco>
References: <20181114114400.15577-1-pbonzini@redhat.com>
	<20181114194208.GA500@flamenco>
	<7648aa01-5458-bef1-9b65-f6ef83287e63@redhat.com>
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In-Reply-To: <7648aa01-5458-bef1-9b65-f6ef83287e63@redhat.com>
Subject: Re: [Qemu-devel] [PATCH] cpus: run work items for all vCPUs if
 single-threaded
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, dgilbert@redhat.com, alex.bennee@linaro.org

On Fri, Nov 16, 2018 at 00:15:53 +0100, Paolo Bonzini wrote:
> On 14/11/2018 20:42, Emilio G. Cota wrote:
> > On Wed, Nov 14, 2018 at 12:44:00 +0100, Paolo Bonzini wrote:
> >> This avoids the following deadlock:
> >>
> >> 1) a thread calls run_on_cpu for CPU 2 from a timer, and single_tcg_halt_cond
> >> is signaled
> >>
> >> 2) CPU 1 is running and exits.  It finds no work item and enters CPU 2
> >>
> >> 3) because the I/O thread is stuck in run_on_cpu, the round-robin kick
> >> timer never triggers, and CPU 2 never runs the work item
> >>
> >> 4) run_on_cpu never completes
(snip)
> > I can see though how with an additional CPU the deadlock
> > could happen. For example, the I/O thread does run_on_cpu(cpu3),
> > which kicks cpu1 (i.e. the tcg_current_rr_cpu) and cpu3, but not cpu2.
> > Then cpu1 exits, and cpu2 starts executing; unless cpu2 exits on its
> > own volition, it will run forever.
> 
> Yes, the thread must call run_on_cpu for CPU *3* from a timer.

Thanks! Please add my

Reviewed-by: Emilio G. Cota <cota@braap.org>

tag when fixing up the commit message.

		E.