linux-kernel - Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function

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Message-ID: <20090218170517.GA9769@elte.hu>
Date:	Wed, 18 Feb 2009 18:05:17 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Yinghai Lu <yinghai@...nel.org>, Nick Piggin <npiggin@...e.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jens Axboe <jens.axboe@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
	single ipi fallback for smp_call_function_many())


an off-list comment pointed out this piece of information as 
well:

  http://www.sandpile.org/ia32/coherent.htm

  A WRMSR to one of the x2APIC MSRs (0000_0800h...0000_0BFFh) is 
  not guaranteed to be serializing.

So i suspect we should just enclose it in smp_mb() pairs to make 
sure it's a full barrier in both directions?

Although since it's an explicitly async mechanism with no 
ordering expectations whatsoever, just doing an smp_mb() before 
it should be enough, to make sure the IPI can never arrive to 
another CPU/core faster than the event-horizon/visibility of 
preceding operations.

	Ingo
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