Message-ID: <439DD082.9508C87C@tv-sign.ru>
Date: Mon, 12 Dec 2005 22:33:22 +0300
From: Oleg Nesterov <oleg@tv-sign.ru>
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To: vatsa@in.ibm.com
Cc: Keith Owens <kaos@ocs.com.au>, Rusty Russell <rusty@rustcorp.com.au>,
       ajwade@cpe001346162bf9-cm0011ae8cd564.cpe.net.cable.rogers.com,
       linux-kernel@vger.kernel.org, Dipankar Sarma <dipankar@in.ibm.com>,
       "Paul E. McKenney" <paulmck@us.ibm.com>, Andrew Morton <akpm@osdl.org>,
       Ingo Molnar <mingo@elte.hu>
Subject: Re: Semantics of smp_mb() [was : Re: [PATCH] Fix RCU race in access of 
 nohz_cpu_mask ]
References: <1134344716.5937.11.camel@localhost.localdomain> <18382.1134348547@ocs3.ocs.com.au> <20051212084112.GA3934@in.ibm.com>
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Srivatsa Vaddagiri wrote:
> 
> Oleg, with all these inputs, I consider the patch I had sent to be correct.

Yes, I think so. My suspects were due to my misunderstanding.
I was wrong when I said that we can ignore rmb() part of mb()
in case when start_hz_timer() runs after rcu_start_batch().

	rcp->cur++;
	// ->cur will be visible to other cpus
	// _before_ we will *READ* nohz_cpu_mask.
	// we don't have any 'timing' problems.
	// In other words: if another cpu does not
	// see the new value - we did not read this
	// mask yet.
	smp_mb();

Thanks for your patience.

> P.S :- Thanks to everybody who reponded clarifying this subject.

Yes! it was really helpful, thanks to all. I think it would be great
to have Paul's very clear (and short!) explanation somewhere in
Documentation/.

Oleg.
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