From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758899AbbDYJny (ORCPT ); Sat, 25 Apr 2015 05:43:54 -0400 Received: from e06smtp16.uk.ibm.com ([195.75.94.112]:41982 "EHLO e06smtp16.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754606AbbDYJnw (ORCPT ); Sat, 25 Apr 2015 05:43:52 -0400 Date: Sat, 25 Apr 2015 11:43:46 +0200 From: Heiko Carstens To: Rik van Riel Cc: linux-kernel@vger.kernel.org, Andy Lutomirsky , Frederic Weisbecker , Peter Zijlstra , williams@redhat.com Subject: Re: [PATCH v2] context_tracking: remove local_irq_save from __acct_update_integrals Message-ID: <20150425094346.GA5897@osiris> References: <20150424111653.2a87a103@annuminas.surriel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150424111653.2a87a103@annuminas.surriel.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15042509-0025-0000-0000-000004E178D6 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 24, 2015 at 11:16:53AM -0400, Rik van Riel wrote: > V2: introduce signed_cputime_t to deal with 64 bit cputime_t on > 32 bit architectures, and use READ_ONCE to ensure the value > is always read atomically (Heiko Karstens) Erm, that's not what I said ;) READ_ONCE() only fixes the isssue that with your previous code the compiler was free to generate code that accesses the memory value several times. But.. > - local_irq_save(flags); > time = stime + utime; > - dtime = time - tsk->acct_timexpd; > + dtime = time - READ_ONCE(tsk->acct_timexpd); > + /* > + * This code is called both from irq context and from > + * task context. There is a race where irq context advances > + * tsk->acct_timexpd to a value larger than time, creating > + * a negative value. In that case, the irq has already > + * updated the statistics. > + */ > + if (unlikely((signed_cputime_t)dtime <= 0)) > + return; > + ...the READ_ONCE() doesn't give you any guarantees about reading tsk->acct_timexpd in an atomic way. Well, actually you don't need atomic semantics, but only to make sure that the read access happens with a single instruction, since you want to protect against interrupts. But still: if the size of acct_timexpd is 64 bit READ_ONCE() may still result in two instructions on 32 bit architectures. (or isn't there currently no 32 bit architecture with 64 bit cputime_t left?)