linux-kernel - Re: [RFC][PATCH v2] sched/rt: Use IPI to trigger RT task push migration instead of pulling

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Date:	Thu, 26 Feb 2015 08:45:59 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jörn Engel <joern@...estorage.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC][PATCH v2] sched/rt: Use IPI to trigger RT task push
 migration instead of pulling

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.
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