From patchwork Sat Nov 11 13:26:31 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Byungchul Park X-Patchwork-Id: 10054283 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 6757B60632 for ; Sat, 11 Nov 2017 13:27:19 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 50E9D29B3F for ; Sat, 11 Nov 2017 13:27:19 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 4432229B44; Sat, 11 Nov 2017 13:27:19 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id CD33829B3F for ; Sat, 11 Nov 2017 13:27:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750953AbdKKN0i (ORCPT ); Sat, 11 Nov 2017 08:26:38 -0500 Received: from LGEAMRELO13.lge.com ([156.147.23.53]:38039 "EHLO lgeamrelo13.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750818AbdKKN0g (ORCPT ); Sat, 11 Nov 2017 08:26:36 -0500 Received: from unknown (HELO lgeamrelo04.lge.com) (156.147.1.127) by 156.147.23.53 with ESMTP; 11 Nov 2017 22:26:34 +0900 X-Original-SENDERIP: 156.147.1.127 X-Original-MAILFROM: byungchul.park@lge.com Received: from unknown (HELO localhost.localdomain) (10.177.222.33) by 156.147.1.127 with ESMTP; 11 Nov 2017 22:26:34 +0900 X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com From: Byungchul Park To: peterz@infradead.org, mingo@kernel.org Cc: tglx@linutronix.de, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-block@vger.kernel.org, kernel-team@lge.com Subject: [PATCH v3 4/5] locking/Documentation: Add an example to help crossrelease.txt more readable Date: Sat, 11 Nov 2017 22:26:31 +0900 Message-Id: <1510406792-28676-5-git-send-email-byungchul.park@lge.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1510406792-28676-1-git-send-email-byungchul.park@lge.com> References: <1510406792-28676-1-git-send-email-byungchul.park@lge.com> Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Add an example explaining the rationale that the limitation that old lockdep implies, can be relaxed. Signed-off-by: Byungchul Park --- Documentation/locking/crossrelease.txt | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+) diff --git a/Documentation/locking/crossrelease.txt b/Documentation/locking/crossrelease.txt index bb449e8..dac56f4 100644 --- a/Documentation/locking/crossrelease.txt +++ b/Documentation/locking/crossrelease.txt @@ -281,6 +281,29 @@ causes a deadlock. The more lockdep adds dependencies, the more it thoroughly works. Thus, lockdep has to do its best to detect and add as many true dependencies to the graph as possible. +For example: + + CONTEXT X CONTEXT Y + --------- --------- + acquire A + acquire B /* A dependency 'A -> B' exists */ + release B + release A held by Y + + where A and B are different lock classes. + +In this case, a dependency 'A -> B' exists since: + + 1. A waiter for A and a waiter for B might exist when acquiring B. + 2. The only way to wake up each is to release what it waits for. + 3. Whether the waiter for A can be woken up depends on whether the + other can. In other words, CONTEXT X cannot release A if it fails + to acquire B. + +Considering only typical locks, lockdep builds nothing. However, +relaxing the limitation, a dependency 'A -> B' can be added, giving us +more chances to check circular dependencies. + However, it might suffer performance degradation since relaxing the limitation, with which design and implementation of lockdep can be efficient, might introduce inefficiency inevitably. So lockdep