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Date: Thu, 26 Feb 2015 08:45:59 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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        =?iso-8859-1?Q?J=F6rn?= Engel <joern@purestorage.com>,
        Oleg Nesterov <oleg@redhat.com>
Subject: Re: [RFC][PATCH v2] sched/rt: Use IPI to trigger RT task push
 migration instead of pulling
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On Wed, Feb 25, 2015 at 12:50:15PM -0500, Steven Rostedt wrote:
> It can't be used for state?
> 
> If one CPU writes "zero", and the other CPU wants to decide if the
> system is in the state to do something, isn't a rmb() fine to use?
> 
> 
> CPU 1:
> 
> 	x = 0;
> 	/* Tell other CPUs they can now do something */
> 	smp_wmb();
> 
> CPU 2:
> 	/* Make sure we see current state of x */
> 	smp_rmb();
> 	if (x == 0)
> 		do_something();
> 
> The above situation is not acceptable?

Acceptable is just not the word. It plain doesn't work that way.

> Otherwise, we fail to be able to do_something() when it is perfectly
> fine to do so.

Can't be helped.