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

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Message-ID: <20131029103526.GO2490@laptop.programming.kicks-ass.net>
Date:	Tue, 29 Oct 2013 11:35:26 +0100
From:	Peter Zijlstra <peterz@...radead.org>
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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: perf events ring buffer memory barrier on powerpc

On Tue, Oct 29, 2013 at 11:30:57AM +0100, Peter Zijlstra wrote:
> @@ -154,9 +175,11 @@ int perf_output_begin(struct perf_output
>  		 * Userspace could choose to issue a mb() before updating the
>  		 * tail pointer. So that all reads will be completed before the
>  		 * write is issued.
> +		 *
> +		 * See perf_output_put_handle().
>  		 */
>  		tail = ACCESS_ONCE(rb->user_page->data_tail);
> -		smp_rmb();
> +		smp_mb();
>  		offset = head = local_read(&rb->head);
>  		head += size;
>  		if (unlikely(!perf_output_space(rb, tail, offset, head)))

That said; it would be very nice to be able to remove this barrier. This
is in every event write path :/
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