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=-16.6 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_GIT,USER_IN_DEF_DKIM_WL 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 682D7C04AB4 for ; Wed, 15 May 2019 00:32:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3612520862 for ; Wed, 15 May 2019 00:32:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="v/iBtmpp" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727023AbfEOAcJ (ORCPT ); Tue, 14 May 2019 20:32:09 -0400 Received: from mail-pf1-f202.google.com ([209.85.210.202]:46696 "EHLO mail-pf1-f202.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726218AbfEOAcI (ORCPT ); Tue, 14 May 2019 20:32:08 -0400 Received: by mail-pf1-f202.google.com with SMTP id d9so456356pfo.13 for ; Tue, 14 May 2019 17:32:08 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:message-id:mime-version:subject:from:to:cc; bh=RTskpJzrqjSfKamQuryjYgpBB8UNg8rBhhyvKDtP8Jc=; b=v/iBtmppsGFx8fleVlGl5FyzcENGHfuP9fHcZoab0NyXIBDYPrxa5R32Jvgpl7IU6F xXjqQZdjrTjRiyYIwg1LJfSqon7Gau5FSrRXq7C84LbOwMIC/DyhUk23yNOdp6647IQj pJHSJlvQD8aItNKPUtiF4HEz14o0wFtx7cDJ1fgmKWvokxJma08MNDOeRRtSsSSJm5VV vxIlYi3Z0x8CfQJS0gcZ7kVdnCRkbt7tVhJx3w1hEAEMWq5HQctSS2j9xy4lqNiq7K80 mXoaW5Ytl9RJfZYE8A1Qk3ZF/iauoztRI0B4B8d21HU+sXtfhrH6fj0R0o1kcY+zwOJa oLJw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:message-id:mime-version:subject:from:to:cc; bh=RTskpJzrqjSfKamQuryjYgpBB8UNg8rBhhyvKDtP8Jc=; b=m4ooY08/MqXpumpiXfzvpsqwC7qh4CaGiJ0ORqizfPVE5qgzp/3mkBOtIqD8Z/G66C W9arQrFVaxWMm8ywxKl5mHtblDX9W033iZcOrBwmx1WCWaThVIcTFZfMwmX36io8lW7h aBfD31dLNjYyp9V3Ks0E4lwFNbOFOI/WjJXPOAA+w7Q6fg5zmcgVHdlH3eUaX1PF0gUM /2n1PskHR8e5AJCSlwOtt58p/slxz0qe6mND+sQISOK2DPT1k41fC1URIoONX5JKiqUW H+9pwH7qocig14hGdy96CEqMtFCnBEIYb3rA7GeTMYeN2U3taKanJciKj+QSrbD2nxpL H+7g== X-Gm-Message-State: APjAAAUnJofggfPLlw4YYhSoXW2rMpzZLiH0TeTkmlOEGQexQPlBi02o EK56dsF9NBma1VzJMbFhYyMBrMbnJg== X-Google-Smtp-Source: APXvYqxXVEDKuBgb5OQ4XZpudHbVkE8PutAYv0Z8IBFEIzSl+Wtc/MG9h9QzPY0Stjw+V/+iaO6JvwQotSs= X-Received: by 2002:a63:4006:: with SMTP id n6mr41416529pga.424.1557880327498; Tue, 14 May 2019 17:32:07 -0700 (PDT) Date: Tue, 14 May 2019 17:30:59 -0700 Message-Id: <20190515003059.23920-1-yabinc@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.21.0.1020.gf2820cf01a-goog Subject: [PATCH] perf/ring_buffer: Fix exposing a temporarily decreased data_head. From: Yabin Cui To: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , Jiri Olsa , Namhyung Kim Cc: linux-kernel@vger.kernel.org, Yabin Cui Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org In perf_output_put_handle(), an IRQ/NMI can happen in below location and write records to the same ring buffer: ... local_dec_and_test(&rb->nest) ... <-- an IRQ/NMI can happen here rb->user_page->data_head = head; ... In this case, a value A is written to data_head in the IRQ, then a value B is written to data_head after the IRQ. And A > B. As a result, data_head is temporarily decreased from A to B. And a reader may see data_head < data_tail if it read the buffer frequently enough, which creates unexpected behaviors. This can be fixed by moving dec(&rb->nest) to after updating data_head, which prevents the IRQ/NMI above from updating data_head. Signed-off-by: Yabin Cui --- kernel/events/ring_buffer.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index 674b35383491..0b9aefe13b04 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -54,8 +54,10 @@ static void perf_output_put_handle(struct perf_output_handle *handle) * IRQ/NMI can happen here, which means we can miss a head update. */ - if (!local_dec_and_test(&rb->nest)) + if (local_read(&rb->nest) > 1) { + local_dec(&rb->nest); goto out; + } /* * Since the mmap() consumer (userspace) can run on a different CPU: @@ -86,6 +88,13 @@ static void perf_output_put_handle(struct perf_output_handle *handle) smp_wmb(); /* B, matches C */ rb->user_page->data_head = head; + /* + * Clear rb->nest after updating data_head. This prevents IRQ/NMI from + * updating data_head before us. If that happens, we will expose a + * temporarily decreased data_head. + */ + local_set(&rb->nest, 0); + /* * Now check if we missed an update -- rely on previous implied * compiler barriers to force a re-read. -- 2.21.0.1020.gf2820cf01a-goog