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From: Ingo Molnar <mingo@elte.hu>
To: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: jens.axboe@oracle.com, paulmck@linux.vnet.ibm.com,
	jeremy.fitzhardinge@citrix.com, nickpiggin@yahoo.com.au,
	torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
	asit.k.mallick@intel.com
Subject: Re: [patch] generic-ipi: fix the smp_mb() placement
Date: Thu, 30 Oct 2008 19:53:22 +0100	[thread overview]
Message-ID: <20081030185322.GE17822@elte.hu> (raw)
In-Reply-To: <20081029224229.GK30573@linux-os.sc.intel.com>


* Suresh Siddha <suresh.b.siddha@intel.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? Seems like a v2.6.28 
fix to me in any case.

	Ingo

  parent reply	other threads:[~2008-10-30 18:54 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-10-29 22:42 Suresh Siddha
2008-10-30  7:20 ` Jens Axboe
2008-10-30 16:30   ` Suresh Siddha
2008-10-30 17:25     ` Jens Axboe
2008-10-30 18:53 ` Ingo Molnar [this message]
2008-10-30 20:23   ` Suresh Siddha
2008-10-31  5:10     ` Jeremy Fitzhardinge
2008-10-31  9:39       ` Ingo Molnar
2008-10-31 11:12         ` Jeremy Fitzhardinge
2008-10-31 16:53           ` Suresh Siddha
2008-10-31 20:30             ` Jeremy Fitzhardinge
2008-11-03 10:17               ` Ingo Molnar
2008-11-03 23:48                 ` Jeremy Fitzhardinge
2008-11-04  9:19                   ` Ingo Molnar
2008-11-04 22:25                     ` Suresh Siddha
2008-11-05  9:20                       ` Jens Axboe

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