linux-kernel - Re: perf events ring buffer memory barrier on powerpc

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Message-ID: <20131030092725.GL4126@linux.vnet.ibm.com>
Date:	Wed, 30 Oct 2013 02:27:25 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Victor Kaplansky <VICTORK@...ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Anton Blanchard <anton@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PPC dev <linuxppc-dev@...abs.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Michael Ellerman <michael@...erman.id.au>,
	Michael Neuling <mikey@...ling.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: perf events ring buffer memory barrier on powerpc

On Mon, Oct 28, 2013 at 10:58:58PM +0200, Victor Kaplansky wrote:
> Oleg Nesterov <oleg@...hat.com> wrote on 10/28/2013 10:17:35 PM:
> 
> >       mb();   // XXXXXXXX: do we really need it? I think yes.
> 
> Oh, it is hard to argue with feelings. Also, it is easy to be on
> conservative side and put the barrier here just in case.
> But I still insist that the barrier is redundant in your example.

If you were to back up that insistence with a description of the orderings
you are relying on, why other orderings are not important, and how the
important orderings are enforced, I might be tempted to pay attention
to your opinion.

							Thanx, Paul

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