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=-5.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 A22EDC43441 for ; Thu, 22 Nov 2018 15:05:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7092A20685 for ; Thu, 22 Nov 2018 15:05:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7092A20685 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2405734AbeKWBpN (ORCPT ); Thu, 22 Nov 2018 20:45:13 -0500 Received: from mx1.redhat.com ([209.132.183.28]:47674 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726758AbeKWBpM (ORCPT ); Thu, 22 Nov 2018 20:45:12 -0500 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id B589C3164983; Thu, 22 Nov 2018 15:05:27 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.142]) by smtp.corp.redhat.com (Postfix) with SMTP id 09B615C21E; Thu, 22 Nov 2018 15:05:25 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Thu, 22 Nov 2018 16:05:27 +0100 (CET) Date: Thu, 22 Nov 2018 16:05:24 +0100 From: Oleg Nesterov To: Andrea Parri Cc: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , Jiri Olsa , Namhyung Kim , linux-kernel@vger.kernel.org Subject: Re: [Question] smp_wmb() in prepare_uprobe() Message-ID: <20181122150524.GF28270@redhat.com> References: <20181121224124.GB4016@andrea> <20181122123655.GD28270@redhat.com> <20181122134400.GA10327@andrea> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181122134400.GA10327@andrea> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Thu, 22 Nov 2018 15:05:28 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/22, Andrea Parri wrote: > > > See 142b18ddc81439acda4bc4231b291e99fe67d507 ("uprobes: Fix handle_swbp() > > vs unregister() + register() race") and the comment above this rmb(). > > Mmh..., at first glance, this suggests me that the above set_bit() and > test_bit() to/from uprobe->flags are among these memory accesses. But > this doesn't make sense to me: these accesses do not "alternate" (i.e., > they both appear after the corresponding barrier..); instead I'd expect > something like (on top of the above diff): > > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c > index 2d29977522017..a75b9a08dee54 100644 > --- a/kernel/events/uprobes.c > +++ b/kernel/events/uprobes.c > @@ -2178,10 +2178,18 @@ static void handle_swbp(struct pt_regs *regs) > * After we hit the bp, _unregister + _register can install the > * new and not-yet-analyzed uprobe at the same address, restart. > */ > - smp_rmb(); /* pairs with the smp_wmb() in prepare_uprobe() */ > if (unlikely(!test_bit(UPROBE_COPY_INSN, &uprobe->flags))) > goto out; > > + /* > + * Pairs with the smp_wmb() in prepare_uprobe(). > + * > + * Guarantees that if we see the UPROBE_COPY_INSN bit set, then > + * we must (can) also see the stores to &uprobe->arch performed > + * by prepare_uprobe() (say). > + */ > + smp_rmb(); OOPS, you are right! Thanks. Oleg.