From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755240AbcEQOWW (ORCPT ); Tue, 17 May 2016 10:22:22 -0400 Received: from terminus.zytor.com ([198.137.202.10]:40956 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750875AbcEQOWV (ORCPT ); Tue, 17 May 2016 10:22:21 -0400 Date: Tue, 17 May 2016 07:21:40 -0700 From: tip-bot for Peter Zijlstra Message-ID: Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, markivx@codeaurora.org, peterz@infradead.org, hpa@zytor.com, tglx@linutronix.de, torvalds@linux-foundation.org Reply-To: linux-kernel@vger.kernel.org, mingo@kernel.org, torvalds@linux-foundation.org, tglx@linutronix.de, peterz@infradead.org, hpa@zytor.com, markivx@codeaurora.org In-Reply-To: <20160516131751.GH3205@twins.programming.kicks-ass.net> References: <20160516131751.GH3205@twins.programming.kicks-ass.net> To: linux-tip-commits@vger.kernel.org Subject: [tip:sched/urgent] sched/preempt: Fix preempt_count manipulations Git-Commit-ID: 2e636d5e66c35dfcbaf617aa8fa963f6847478fe X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: 2e636d5e66c35dfcbaf617aa8fa963f6847478fe Gitweb: http://git.kernel.org/tip/2e636d5e66c35dfcbaf617aa8fa963f6847478fe Author: Peter Zijlstra AuthorDate: Mon, 16 May 2016 15:01:11 +0200 Committer: Ingo Molnar CommitDate: Tue, 17 May 2016 12:24:21 +0200 sched/preempt: Fix preempt_count manipulations Vikram reported that his ARM64 compiler managed to 'optimize' away the preempt_count manipulations in code like: preempt_enable_no_resched(); put_user(); preempt_disable(); Irrespective of that fact that that is horrible code that should be fixed for many reasons, it does highlight a deficiency in the generic preempt_count manipulators. As it is never right to combine/elide preempt_count manipulations like this. Therefore sprinkle some volatile in the two generic accessors to ensure the compiler is aware of the fact that the preempt_count is observed outside of the regular program-order view and thus cannot be optimized away like this. x86; the only arch not using the generic code is not affected as we do all this in asm in order to use the segment base per-cpu stuff. Reported-by: Vikram Mulukutla Tested-by: Vikram Mulukutla Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Fixes: a787870924db ("sched, arch: Create asm/preempt.h") Link: http://lkml.kernel.org/r/20160516131751.GH3205@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar --- include/asm-generic/preempt.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h index 5d8ffa3..c1cde35 100644 --- a/include/asm-generic/preempt.h +++ b/include/asm-generic/preempt.h @@ -7,10 +7,10 @@ static __always_inline int preempt_count(void) { - return current_thread_info()->preempt_count; + return READ_ONCE(current_thread_info()->preempt_count); } -static __always_inline int *preempt_count_ptr(void) +static __always_inline volatile int *preempt_count_ptr(void) { return ¤t_thread_info()->preempt_count; }