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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, 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 2A5AFC43219 for ; Fri, 3 May 2019 15:37:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 055592081C for ; Fri, 3 May 2019 15:37:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728325AbfECPhy (ORCPT ); Fri, 3 May 2019 11:37:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41532 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726476AbfECPhy (ORCPT ); Fri, 3 May 2019 11:37:54 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 521BC307EA9C; Fri, 3 May 2019 15:37:54 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.159]) by smtp.corp.redhat.com (Postfix) with SMTP id 0903819C4F; Fri, 3 May 2019 15:37:48 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Fri, 3 May 2019 17:37:54 +0200 (CEST) Date: Fri, 3 May 2019 17:37:48 +0200 From: Oleg Nesterov To: Peter Zijlstra Cc: Sebastian Andrzej Siewior , linux-rt-users , LKML , Thomas Gleixner , Daniel Bristot de Oliveira , Clark Williams , Juri Lelli , jack@suse.com, Waiman Long , Davidlohr Bueso Subject: Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16) Message-ID: <20190503153747.GC20802@redhat.com> References: <20190326093421.GA29508@localhost.localdomain> <20190419085627.GI4742@localhost.localdomain> <20190430125130.uw7mhdnsoqr2v3gf@linutronix.de> <20190430132811.GB2589@hirez.programming.kicks-ass.net> <20190501170953.GB2650@hirez.programming.kicks-ass.net> <20190502100932.GA7323@redhat.com> <20190503141633.GB2606@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190503141633.GB2606@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Fri, 03 May 2019 15:37:54 +0000 (UTC) Sender: linux-rt-users-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org On 05/03, Peter Zijlstra wrote: > > On Thu, May 02, 2019 at 12:09:32PM +0200, Oleg Nesterov wrote: > > > > +static void readers_block(struct percpu_rw_semaphore *sem) > > > +{ > > > + wait_event_cmd(sem->writer, !sem->readers_block, > > > + __up_read(&sem->rw_sem), __down_read(&sem->rw_sem)); > > > +} > > > + > > > +static void block_readers(struct percpu_rw_semaphore *sem) > > > +{ > > > + wait_event_exclusive_cmd(sem->writer, !sem->readers_block, > > > + __up_write(&sem->rw_sem), > > > + __down_write(&sem->rw_sem)); > > > + /* > > > + * Notify new readers to block; up until now, and thus throughout the > > > + * longish rcu_sync_enter() above, new readers could still come in. > > > + */ > > > + WRITE_ONCE(sem->readers_block, 1); > > > +} > > > > So iiuc, despite it name block_readers() also serializes the writers, ->rw_sem > > can be dropped by down_write_non_owner() so the new writer can take this lock. > > I don't think block_readers() is sufficient to serialize writers; > suppose two concurrent callers when !->readers_block. Without ->rwsem > that case would not serialize. Of course. I meant that the next writer can enter block_readers() if up_non_owner() drops ->rw_sem, but it will block in wait_event(!readers_block). (And if we change this code to use wait_event(xchg(readers_block) == 0) we can remove rw_sem altogether). The main problem is that this is sub-optimal. We can have a lot of readers sleeping in __down_read() when percpu_down_write() succeeds, then after percpu_down_write_non_owner() does up_write() they all will be woken just to hang in readers_block(). Plus the new readers will need to pass the lock-check-unlock-schedule path. Peter, just in case... I see another patch from you but I need to run away till Monday. Oleg.