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 2BD65C433FE for ; Fri, 30 Sep 2022 04:50:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230352AbiI3Eu0 (ORCPT ); Fri, 30 Sep 2022 00:50:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54176 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230351AbiI3EuV (ORCPT ); Fri, 30 Sep 2022 00:50:21 -0400 Received: from alexa-out-sd-02.qualcomm.com (alexa-out-sd-02.qualcomm.com [199.106.114.39]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7DFF11A803 for ; Thu, 29 Sep 2022 21:49:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=quicinc.com; i=@quicinc.com; q=dns/txt; s=qcdkim; t=1664513388; x=1696049388; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=/KfZ4OUR1AljEZyjACuGNyt+cwoWa/eMPZugI7IrO4A=; b=HD//JKn/8FV14M7P866dyJagT0tNSdK4cns71N/15Tk3uR6jzC+eZg0n uSAu6/kF8V0TOVXOo29Tx2Dc7TpKOLtEAJ39WT0wD/p+sPWIV4e4d9xnZ F2cJmrp3UARqbQrrLCBlvpDOZYMA9wzbMgTu5FHCCUMhWtmpdHtfPfd+s k=; Received: from unknown (HELO ironmsg-SD-alpha.qualcomm.com) ([10.53.140.30]) by alexa-out-sd-02.qualcomm.com with ESMTP; 29 Sep 2022 21:46:48 -0700 X-QCInternal: smtphost Received: from nasanex01c.na.qualcomm.com ([10.45.79.139]) by ironmsg-SD-alpha.qualcomm.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Sep 2022 21:46:48 -0700 Received: from [10.216.24.223] (10.80.80.8) by nasanex01c.na.qualcomm.com (10.45.79.139) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.986.29; Thu, 29 Sep 2022 21:46:45 -0700 Message-ID: <0e0a6a29-1b2a-ff86-123d-1f6da272879c@quicinc.com> Date: Fri, 30 Sep 2022 10:16:41 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.13.0 Subject: Re: [PATCH] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath Content-Language: en-US To: Waiman Long , Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng CC: , , Hillf Danton References: <20220929180413.107374-1-longman@redhat.com> From: Mukesh Ojha In-Reply-To: <20220929180413.107374-1-longman@redhat.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.80.80.8] X-ClientProxiedBy: nasanex01a.na.qualcomm.com (10.52.223.231) To nasanex01c.na.qualcomm.com (10.45.79.139) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Thanks for looking into this issue. On 9/29/2022 11:34 PM, 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(-) > > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c > index 65f0262f635e..ad676e99e0b3 100644 > --- a/kernel/locking/rwsem.c > +++ b/kernel/locking/rwsem.c > @@ -628,6 +628,11 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, > new = count; > > if (count & RWSEM_LOCK_MASK) { > + /* > + * A waiter (first or not) can set the handoff bit > + * if it is an RT task or wait in the wait queue > + * for too long. > + */ > if (has_handoff || (!rt_task(waiter->task) && > !time_after(jiffies, waiter->timeout))) > return false; > @@ -643,11 +648,13 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, > } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new)); > > /* > - * We have either acquired the lock with handoff bit cleared or > - * set the handoff bit. > + * We have either acquired the lock with handoff bit cleared or set > + * the handoff bit. Only the first waiter can have its handoff_set > + * set here to enable optimistic spinning in slowpath loop. > */ > if (new & RWSEM_FLAG_HANDOFF) { > - waiter->handoff_set = true; > + if (waiter == first) > + waiter->handoff_set = true; > lockevent_inc(rwsem_wlock_handoff); nit: Should this not get incremented on waiter->handoff_set=true only ? > return false; > } -Mukesh