Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752059Ab3JWOZr (ORCPT ); Wed, 23 Oct 2013 10:25:47 -0400 Received: from mail-wg0-f45.google.com ([74.125.82.45]:47840 "EHLO mail-wg0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751531Ab3JWOZp (ORCPT ); Wed, 23 Oct 2013 10:25:45 -0400 MIME-Version: 1.0 In-Reply-To: <20131023141948.GB3566@localhost.localdomain> References: <12083.1382486094@ale.ozlabs.ibm.com> <20131023141948.GB3566@localhost.localdomain> Date: Wed, 23 Oct 2013 15:25:44 +0100 Message-ID: Subject: Re: perf events ring buffer memory barrier on powerpc From: Frederic Weisbecker To: Michael Neuling , Peter Zijlstra Cc: Benjamin Herrenschmidt , Anton Blanchard , LKML , Linux PPC dev , Victor Kaplansky , Mathieu Desnoyers , michael Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2638 Lines: 73 2013/10/23 Frederic Weisbecker : > On Wed, Oct 23, 2013 at 10:54:54AM +1100, Michael Neuling wrote: >> Frederic, >> >> In the perf ring buffer code we have this in perf_output_get_handle(): >> >> if (!local_dec_and_test(&rb->nest)) >> goto out; >> >> /* >> * Publish the known good head. Rely on the full barrier implied >> * by atomic_dec_and_test() order the rb->head read and this >> * write. >> */ >> rb->user_page->data_head = head; >> >> The comment says atomic_dec_and_test() but the code is >> local_dec_and_test(). >> >> On powerpc, local_dec_and_test() doesn't have a memory barrier but >> atomic_dec_and_test() does. Is the comment wrong, or is >> local_dec_and_test() suppose to imply a memory barrier too and we have >> it wrongly implemented in powerpc? >> >> My guess is that local_dec_and_test() is correct but we to add an >> explicit memory barrier like below: >> >> (Kudos to Victor Kaplansky for finding this) >> >> Mikey >> >> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c >> index cd55144..95768c6 100644 >> --- a/kernel/events/ring_buffer.c >> +++ b/kernel/events/ring_buffer.c >> @@ -87,10 +87,10 @@ again: >> goto out; >> >> /* >> - * Publish the known good head. Rely on the full barrier implied >> - * by atomic_dec_and_test() order the rb->head read and this >> - * write. >> + * Publish the known good head. We need a memory barrier to order the >> + * order the rb->head read and this write. >> */ >> + smp_mb (); >> rb->user_page->data_head = head; >> >> /* > > > I'm adding Peter in Cc since he wrote that code. > I agree that local_dec_and_test() doesn't need to imply an smp barrier. > All it has to provide as a guarantee is the atomicity against local concurrent > operations (interrupts, preemption, ...). > > Now I'm a bit confused about this barrier. > > I think we want this ordering: > > Kernel User > > READ rb->user_page->data_tail READ rb->user_page->data_head > smp_mb() smp_mb() > WRITE rb data READ rb data > smp_mb() smp_mb() > rb->user_page->data_head WRITE rb->user_page->data_tail ^^ I meant a write above for data_head. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/