From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9BC74C3A5A2 for ; Tue, 3 Sep 2019 20:25:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6EEF721897 for ; Tue, 3 Sep 2019 20:25:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="Hj+GjKX+" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726894AbfICUZC (ORCPT ); Tue, 3 Sep 2019 16:25:02 -0400 Received: from merlin.infradead.org ([205.233.59.134]:56046 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725977AbfICUZC (ORCPT ); Tue, 3 Sep 2019 16:25:02 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=p20SyTx54rQ/rC+3qpXB42zmxOwM1gRNChHwv7aAJrA=; b=Hj+GjKX+XYxMqwKRCr3w9n0OH HaWSG8j4sBxXXbxzPg4lNXG/rE+AkXLoQ7QOha+I9U3H8FDJH48jzqm+z0rnwuB2y8ruG8L+iZnGt LZQ1bwk5JomqBCBeNo20IrgZcfvMYbMpNC9t/RbZ9Sg2ky0p4ECFL3Hu5AVjAqHIg8C+SOjjOmhIX het5zMb36g/vkzCJIG3oC1yuopVxwluJB4blqZ9pdzJbXSLU7iAIYw75UL8355tQU/Ntdh7fvV+TK xBj8XcNwJsGZewyQVRdpO9k+fs5tzcXMWYxCH5e8qgRCl5hAY+M2TzGUatT0RHqA2d4F9XMWbLefk toOtPHU+A==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1i5FLx-0003LC-HJ; Tue, 03 Sep 2019 20:24:38 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 2A6873011DF; Tue, 3 Sep 2019 22:23:58 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 6411420977768; Tue, 3 Sep 2019 22:24:34 +0200 (CEST) Date: Tue, 3 Sep 2019 22:24:34 +0200 From: Peter Zijlstra To: Mathieu Desnoyers Cc: "Paul E. McKenney" , Linus Torvalds , linux-kernel@vger.kernel.org, Oleg Nesterov , "Eric W. Biederman" , Russell King - ARM Linux admin , Chris Metcalf , Christoph Lameter , Kirill Tkhai , Mike Galbraith , Thomas Gleixner , Ingo Molnar Subject: Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load Message-ID: <20190903202434.GX2349@hirez.programming.kicks-ass.net> References: <20190903201135.1494-1-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190903201135.1494-1-mathieu.desnoyers@efficios.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 03, 2019 at 04:11:34PM -0400, Mathieu Desnoyers wrote: > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 9f51932bd543..e24d52a4c37a 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1130,6 +1130,10 @@ struct task_struct { > unsigned long numa_pages_migrated; > #endif /* CONFIG_NUMA_BALANCING */ > > +#ifdef CONFIG_MEMBARRIER > + atomic_t membarrier_state; > +#endif > + > #ifdef CONFIG_RSEQ > struct rseq __user *rseq; > u32 rseq_sig; > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h > index 4a7944078cc3..3577cd7b3dbb 100644 > --- a/include/linux/sched/mm.h > +++ b/include/linux/sched/mm.h > @@ -371,7 +371,17 @@ static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm) > static inline void membarrier_execve(struct task_struct *t) > { > atomic_set(&t->mm->membarrier_state, 0); > + atomic_set(&t->membarrier_state, 0); > } > + > +static inline void membarrier_prepare_task_switch(struct task_struct *t) > +{ > + if (!t->mm) > + return; > + atomic_set(&t->membarrier_state, > + atomic_read(&t->mm->membarrier_state)); > +} > + > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 010d578118d6..8d4f1f20db15 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3038,6 +3038,7 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev, > perf_event_task_sched_out(prev, next); > rseq_preempt(prev); > fire_sched_out_preempt_notifiers(prev, next); > + membarrier_prepare_task_switch(next); > prepare_task(next); > prepare_arch_switch(next); > } Yuck yuck yuck.. so the problem I have with this is that we add yet another cacheline :/ Why can't we frob this state into a line/word we already have to unconditionally touch, like the thread_info::flags word for example. The above also does the store unconditionally, even though, in the most common case, it won't have to.