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 D06F6C4332F for ; Mon, 10 Oct 2022 16:57:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229814AbiJJQ5r (ORCPT ); Mon, 10 Oct 2022 12:57:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43876 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229451AbiJJQ5p (ORCPT ); Mon, 10 Oct 2022 12:57:45 -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 AC4154661C for ; Mon, 10 Oct 2022 09:57:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1665421063; 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=5S+BNyIeJIXqDBwU61BSD8ffgXS2PZduRFuxFNybg6Q=; b=K87v/b9Tb3Fw8nYNAnVBY2BF2MVvASpCUm8N0oH8v0AXWmgxCH68LFi5iqcGr865GYaUUD id3+M5p1t0b8DOYMCFG3LvN0xLg/iyh6PYwXiY4HsIGS3RnroD8EPJlHV7LpO4EJRocRZw qYbbyjsiIPWUA7Uso2FCVKhuGUf0XGE= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-481-pq7lrMkpOVyOwIR4Qn4_QA-1; Mon, 10 Oct 2022 12:57:37 -0400 X-MC-Unique: pq7lrMkpOVyOwIR4Qn4_QA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 1CC57101A528; Mon, 10 Oct 2022 16:57:37 +0000 (UTC) Received: from [10.22.33.72] (unknown [10.22.33.72]) by smtp.corp.redhat.com (Postfix) with ESMTP id 71CDF40252A9; Mon, 10 Oct 2022 16:57:36 +0000 (UTC) Message-ID: <4c4e5208-ddf5-6191-64b0-54ae0bed1af6@redhat.com> Date: Mon, 10 Oct 2022 12:57:36 -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] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath Content-Language: en-US To: Mukesh Ojha Cc: linux-kernel@vger.kernel.org, john.p.donnelly@oracle.com, Hillf Danton , Peter Zijlstra , Will Deacon , Boqun Feng , Ingo Molnar References: <20220929180413.107374-1-longman@redhat.com> <6530f135-e8ac-bb6c-4715-1ea8f76cf4c4@redhat.com> <7cbf49c9-d122-30e6-68b3-c61eca63e5dc@quicinc.com> From: Waiman Long In-Reply-To: <7cbf49c9-d122-30e6-68b3-c61eca63e5dc@quicinc.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/10/22 06:24, Mukesh Ojha wrote: > Hi Waiman, > > > On 9/29/2022 11:36 PM, Waiman Long wrote: >> On 9/29/22 14:04, 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. >>> >>> Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more >>> consistent") >>> Signed-off-by: Waiman Long >>> --- >>>   kernel/locking/rwsem.c | 13 ++++++++++--- >>>   1 file changed, 10 insertions(+), 3 deletions(-) >> >> Mukesh, can you test if this patch can fix the RT task lockup problem? >> > > Looks like, There is still a window for a race. > > There is a chance when a reader who came first added it's BIAS and > goes to slowpath and before it gets added to wait list it got > preempted by RT task which  goes to slowpath as well and being the > first waiter gets its hand-off bit set and not able to get the lock > due to following condition in rwsem_try_write_lock() > >  630                 if (count & RWSEM_LOCK_MASK) {  ==> reader has > sets its bias > .. > ... > >  634 >  635                         new |= RWSEM_FLAG_HANDOFF; >  636                 } else { >  637                         new |= RWSEM_WRITER_LOCKED; > > > ---------------------->----------------------->------------------------- > > First reader (1)          writer(2) RT task             Lock holder(3) > > It sets > RWSEM_READER_BIAS. > while it is going to > slowpath(as the lock > was held by (3)) and > before it got added > to the waiters list > it got preempted > by (2). >                         RT task also takes >                         the slowpath and add              release the >                         itself into waiting list          rwsem lock >             and since it is the first         clear the >                         it is the next one to get         owner. >                         the lock but it can not >                         get the lock as (count & >                         RWSEM_LOCK_MASK) is set >                         as (1) has added it but >                         not able to remove its >             adjustment. > Good catch! It is indeed a possible livelock scenario. Will update the patch to address that. Thanks, Longman