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 3E78DC04AA8 for ; Thu, 2 May 2019 10:09:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1528A20652 for ; Thu, 2 May 2019 10:09:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726424AbfEBKJp (ORCPT ); Thu, 2 May 2019 06:09:45 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48044 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726266AbfEBKJo (ORCPT ); Thu, 2 May 2019 06:09:44 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id AFAF530832D1; Thu, 2 May 2019 10:09:43 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.159]) by smtp.corp.redhat.com (Postfix) with SMTP id 13EA082783; Thu, 2 May 2019 10:09:34 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Thu, 2 May 2019 12:09:42 +0200 (CEST) Date: Thu, 2 May 2019 12:09:32 +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: <20190502100932.GA7323@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190501170953.GB2650@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Thu, 02 May 2019 10:09:43 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/01, Peter Zijlstra wrote: > > Anyway; I cobbled together the below. Oleg, could you have a look, I'm > sure I messed it up. Oh, I will need to read this carefully. but at first glance I do not see any hole... > +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. And note that the caller of readers_block() does down_read(), the caller of block_readers() does down_write(). So perhaps it makes sense to shift these down_read/write into the helpers above and rename them, void xxx_down_read(struct percpu_rw_semaphore *sem) { __down_read(&sem->rw_sem); wait_event_cmd(sem->writer, !sem->readers_block, __up_read(&sem->rw_sem), __down_read(&sem->rw_sem)); } void xxx_down_write(struct percpu_rw_semaphore *sem) { down_write(&sem->rw_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); } to make this logic more clear? Or even bool ck_read(struct percpu_rw_semaphore *sem) { __down_read(&sem->rw_sem); if (!sem->readers_block) return true; __up_read(&sem->rw_sem); } bool ck_write(struct percpu_rw_semaphore *sem) { down_write(&sem->rw_sem); if (!sem->readers_block) return true; up_write(&sem->rw_sem); } Then percpu_down_read/write can simply do wait_event(ck_read(sem)) and wait_event_exclusive(ck_write(sem)) respectively. But this all is cosmetic, it seems that we can remove ->rw_sem altogether but I am not sure... Oleg.