linux-kernel - Re: [patch] generic-ipi: fix the smp

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Message-ID: <20081030202322.GM30573@linux-os.sc.intel.com>
Date:	Thu, 30 Oct 2008 13:23:23 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"jeremy.fitzhardinge@...rix.com" <jeremy.fitzhardinge@...rix.com>,
	"nickpiggin@...oo.com.au" <nickpiggin@...oo.com.au>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>
Subject: Re: [patch] generic-ipi: fix the smp_mb() placement

On Thu, Oct 30, 2008 at 11:53:22AM -0700, Ingo Molnar wrote:
> 
> * Suresh Siddha <suresh.b.siddha@...el.com> wrote:
> 
> > Subject: generic-ipi: fix the smp_mb() placement
> >
> > smp_mb() is needed (to make the memory operations visible globally)
> > before sending the ipi on the sender and the receiver (on Alpha
> > atleast) needs smp_read_barrier_depends() in the handler before
> > reading the call_single_queue list in a lock-free fashion.
> >
> > On x86, x2apic mode register accesses for sending IPI's don't have
> > serializing semantics. So the need for smp_mb() before sending the
> > IPI becomes more critical in x2apic mode.
> >
> > Remove the unnecessary smp_mb() in csd_flag_wait(), as the presence
> > of that smp_mb() doesn't mean anything on the sender, when the ipi
> > receiver is not doing any thing special (like memory fence) after
> > clearing the CSD_FLAG_WAIT.
> 
> nice! Did you see an actual lockup due to this?

We didn't see the lockup in our tests but Xen folks reported similar failures
with their smp call function code.

> Seems like a v2.6.28 fix to me in any case.

Yes.

thanks,
suresh
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