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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 63296C678D6 for ; Fri, 20 Jan 2023 01:52:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229707AbjATBwF (ORCPT ); Thu, 19 Jan 2023 20:52:05 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55536 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229475AbjATBwE (ORCPT ); Thu, 19 Jan 2023 20:52:04 -0500 Received: from lgeamrelo11.lge.com (lgeamrelo13.lge.com [156.147.23.53]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id D4AE74DCC8 for ; Thu, 19 Jan 2023 17:52:01 -0800 (PST) Received: from unknown (HELO lgeamrelo02.lge.com) (156.147.1.126) by 156.147.23.53 with ESMTP; 20 Jan 2023 10:51:58 +0900 X-Original-SENDERIP: 156.147.1.126 X-Original-MAILFROM: byungchul.park@lge.com Received: from unknown (HELO localhost.localdomain) (10.177.244.38) by 156.147.1.126 with ESMTP; 20 Jan 2023 10:51:58 +0900 X-Original-SENDERIP: 10.177.244.38 X-Original-MAILFROM: byungchul.park@lge.com From: Byungchul Park To: boqun.feng@gmail.com Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, damien.lemoal@opensource.wdc.com, linux-ide@vger.kernel.org, adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org, mingo@redhat.com, peterz@infradead.org, will@kernel.org, tglx@linutronix.de, rostedt@goodmis.org, joel@joelfernandes.org, sashal@kernel.org, daniel.vetter@ffwll.ch, duyuyang@gmail.com, johannes.berg@intel.com, tj@kernel.org, tytso@mit.edu, willy@infradead.org, david@fromorbit.com, amir73il@gmail.com, gregkh@linuxfoundation.org, kernel-team@lge.com, linux-mm@kvack.org, akpm@linux-foundation.org, mhocko@kernel.org, minchan@kernel.org, hannes@cmpxchg.org, vdavydov.dev@gmail.com, sj@kernel.org, jglisse@redhat.com, dennis@kernel.org, cl@linux.com, penberg@kernel.org, rientjes@google.com, vbabka@suse.cz, ngupta@vflare.org, linux-block@vger.kernel.org, paolo.valente@linaro.org, josef@toxicpanda.com, linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk, jack@suse.cz, jlayton@kernel.org, dan.j.williams@intel.com, hch@infradead.org, djwong@kernel.org, dri-devel@lists.freedesktop.org, rodrigosiqueiramelo@gmail.com, melissa.srw@gmail.com, hamohammed.sa@gmail.com, 42.hyeyoo@gmail.com, chris.p.wilson@intel.com, gwan-gyeong.mun@intel.com, max.byungchul.park@gmail.com, longman@redhat.com Subject: Re: [PATCH RFC v7 00/23] DEPT(Dependency Tracker) Date: Fri, 20 Jan 2023 10:51:45 +0900 Message-Id: <1674179505-26987-1-git-send-email-byungchul.park@lge.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: References: Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org Boqun wrote: > On Thu, Jan 19, 2023 at 01:33:58PM +0000, Matthew Wilcox wrote: > > On Thu, Jan 19, 2023 at 03:23:08PM +0900, Byungchul Park wrote: > > > Boqun wrote: > > > > *Looks like the DEPT dependency graph doesn't handle the > > > > fair/unfair readers as lockdep current does. Which bring the > > > > next question. > > > > > > No. DEPT works better for unfair read. It works based on wait/event. So > > > read_lock() is considered a potential wait waiting on write_unlock() > > > while write_lock() is considered a potential wait waiting on either > > > write_unlock() or read_unlock(). DEPT is working perfect for it. > > > > > > For fair read (maybe you meant queued read lock), I think the case > > > should be handled in the same way as normal lock. I might get it wrong. > > > Please let me know if I miss something. > > > > From the lockdep/DEPT point of view, the question is whether: > > > > read_lock(A) > > read_lock(A) > > > > can deadlock if a writer comes in between the two acquisitions and > > sleeps waiting on A to be released. A fair lock will block new > > readers when a writer is waiting, while an unfair lock will allow > > new readers even while a writer is waiting. > > > > To be more accurate, a fair reader will wait if there is a writer > waiting for other reader (fair or not) to unlock, and an unfair reader > won't. What a kind guys, both of you! Thanks. I asked to check if there are other subtle things than this. Fortunately, I already understand what you guys shared. > In kernel there are read/write locks that can have both fair and unfair > readers (e.g. queued rwlock). Regarding deadlocks, > > T0 T1 T2 > -- -- -- > fair_read_lock(A); > write_lock(B); > write_lock(A); > write_lock(B); > unfair_read_lock(A); With the DEPT's point of view (let me re-write the scenario): T0 T1 T2 -- -- -- fair_read_lock(A); write_lock(B); write_lock(A); write_lock(B); unfair_read_lock(A); write_unlock(B); read_unlock(A); read_unlock(A); write_unlock(B); write_unlock(A); T0: read_unlock(A) cannot happen if write_lock(B) is stuck by a B owner not doing either write_unlock(B) or read_unlock(B). In other words: 1. read_unlock(A) happening depends on write_unlock(B) happening. 2. read_unlock(A) happening depends on read_unlock(B) happening. T1: write_unlock(B) cannot happen if unfair_read_lock(A) is stuck by a A owner not doing write_unlock(A). In other words: 3. write_unlock(B) happening depends on write_unlock(A) happening. 1, 2 and 3 give the following dependencies: 1. read_unlock(A) -> write_unlock(B) 2. read_unlock(A) -> read_unlock(B) 3. write_unlock(B) -> write_unlock(A) There's no circular dependency so it's safe. DEPT doesn't report this. > the above is not a deadlock, since T1's unfair reader can "steal" the > lock. However the following is a deadlock: > > T0 T1 T2 > -- -- -- > unfair_read_lock(A); > write_lock(B); > write_lock(A); > write_lock(B); > fair_read_lock(A); > > , since T'1 fair reader will wait. With the DEPT's point of view (let me re-write the scenario): T0 T1 T2 -- -- -- unfair_read_lock(A); write_lock(B); write_lock(A); write_lock(B); fair_read_lock(A); write_unlock(B); read_unlock(A); read_unlock(A); write_unlock(B); write_unlock(A); T0: read_unlock(A) cannot happen if write_lock(B) is stuck by a B owner not doing either write_unlock(B) or read_unlock(B). In other words: 1. read_unlock(A) happening depends on write_unlock(B) happening. 2. read_unlock(A) happening depends on read_unlock(B) happening. T1: write_unlock(B) cannot happen if fair_read_lock(A) is stuck by a A owner not doing either write_unlock(A) or read_unlock(A). In other words: 3. write_unlock(B) happening depends on write_unlock(A) happening. 4. write_unlock(B) happening depends on read_unlock(A) happening. 1, 2, 3 and 4 give the following dependencies: 1. read_unlock(A) -> write_unlock(B) 2. read_unlock(A) -> read_unlock(B) 3. write_unlock(B) -> write_unlock(A) 4. write_unlock(B) -> read_unlock(A) With 1 and 4, there's a circular dependency so DEPT definitely report this as a problem. REMIND: DEPT focuses on waits and events. > FWIW, lockdep is able to catch this (figuring out which is deadlock and > which is not) since two years ago, plus other trivial deadlock detection > for read/write locks. Needless to say, if lib/lock-selftests.c was given > a try, one could find it out on one's own. > > Regards, > Boqun >