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 3BD6BC4332F for ; Thu, 13 Oct 2022 13:33:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229656AbiJMNda (ORCPT ); Thu, 13 Oct 2022 09:33:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41260 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229565AbiJMNd2 (ORCPT ); Thu, 13 Oct 2022 09:33:28 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D117E10692D for ; Thu, 13 Oct 2022 06:33:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1665668005; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=9TlxHg1DEsmNT2KFP6zuBipJtjYUKYkAljx6KzxumBA=; b=cxqedgHp54+VrlTLpJp3XKif3nkm7U297d8P6ZvVmPThR94XLUYQxzBDxmdNgZadfp43OR q0uTxhOZiaDBaGLMlZg9LpSHlYfFs+9YdpgFaPLnzErjh4oI51FkhFycjMhn/qt433T0O2 4lm+uqI4bSk7I9d/CDgzHNRt8Fuufok= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-605-h3vKZ8-NOpyWz8tqG3AgxA-1; Thu, 13 Oct 2022 09:33:19 -0400 X-MC-Unique: h3vKZ8-NOpyWz8tqG3AgxA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 9D4202999B5C; Thu, 13 Oct 2022 13:33:18 +0000 (UTC) Received: from [10.22.17.27] (unknown [10.22.17.27]) by smtp.corp.redhat.com (Postfix) with ESMTP id DF15B1121314; Thu, 13 Oct 2022 13:33:17 +0000 (UTC) Message-ID: Date: Thu, 13 Oct 2022 09:33:17 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH v2 1/2] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath Content-Language: en-US To: Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Boqun Feng , linux-kernel@vger.kernel.org, john.p.donnelly@oracle.com, Hillf Danton , Mukesh Ojha , =?UTF-8?B?VGluZzExIFdhbmcg546L5am3?= References: <20221012133333.1265281-1-longman@redhat.com> <20221012133333.1265281-2-longman@redhat.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/13/22 06:02, Peter Zijlstra wrote: > On Wed, Oct 12, 2022 at 09:33:32AM -0400, Waiman Long wrote: >> A non-first waiter can potentially spin in the for loop of >> rwsem_down_write_slowpath() without sleeping but fail to acquire the >> lock even if the rwsem is free if the following sequence happens: >> >> Non-first waiter First waiter Lock holder >> ---------------- ------------ ----------- >> Acquire wait_lock >> rwsem_try_write_lock(): >> Set handoff bit if RT or >> wait too long >> Set waiter->handoff_set >> Release wait_lock >> Acquire wait_lock >> Inherit waiter->handoff_set >> Release wait_lock >> Clear owner >> Release lock >> if (waiter.handoff_set) { >> rwsem_spin_on_owner((); >> if (OWNER_NULL) >> goto trylock_again; >> } >> trylock_again: >> Acquire wait_lock >> rwsem_try_write_lock(): >> if (first->handoff_set && (waiter != first)) >> return false; >> Release wait_lock >> >> It is especially problematic if the non-first waiter is an RT task and >> it is running on the same CPU as the first waiter as this can lead to >> live lock. >> > So why not do a better handoff? Specifically, have the owner set owner > to first-waiter instead of NULL ? (like the normal mutex code) I understand your desire to make the rwsem handoff process more like what mutex is currently doing. I certainly think it is doable and will put this in my todo list. However, that needs to be done at unlock and wakeup time. I expect that will require moderate amount of code changes which will make it not that suitable for backporting to the stable releases. I would like to see these simple fixes get merged first and then we can work on a major revamp of the handoff code. What do you think? Cheers, Longman