From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 987F61F2C52 for ; Tue, 22 Apr 2025 12:21:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745324502; cv=none; b=Q6xAVLG8NrM/jidWpmEjvy3W8JaW7zcyzKBZ1ki39RKmJnhck9EsLutBUroOaBYrtmNdOCvEGskfCBS6cSNp2q00hpfEySz1Fo5phimQDB/r5dQtkNDRX5PEjSUlKTsuoJ7SNKwVzdbxtVoAL0Qo9o0s9z/7O6Rbjphoy/0Ar+w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745324502; c=relaxed/simple; bh=lf0LhBGzLdmzGwWcHpTy2pWAzeDkex6Gxkosgmr/sBM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=SYrtuA18st+5UQXFBxgfaJFsq4mLWDoEp615nno6FLO9aM7xi8mPKdilL0PU3mOoAlLsLGTGxtp2nExByNHX9ghuf780/wtmf2NAolu96WEFUMkOJEf9rxkpwPVX/nVsj5nFTijOduxN1vN/uFT6chzbdC8R8LXxRdRD97HrYlo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=hW7f/Ol3; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="hW7f/Ol3" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B9DEFC4CEE9; Tue, 22 Apr 2025 12:21:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1745324500; bh=lf0LhBGzLdmzGwWcHpTy2pWAzeDkex6Gxkosgmr/sBM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=hW7f/Ol3TZTXOcGmTJhYrSqCsVlz+NlhXALVOLlxOURNbMx967CzOzzisIBt7+2tn QrgauQTIsqOnoUFb/6wPmlX7mAFwVBgS6lIVbBzrpEiao68pQlzBLaHMy6u4UljiHf yBkLSOPDe9okoBJR4tGFwnOKYpaHJcqSI6vBvFG4= Date: Tue, 22 Apr 2025 14:21:37 +0200 From: Greg KH To: Muchun Song Cc: stable@vger.kernel.org, muchun.song@linux.dev, Ming Lei , Jens Axboe Subject: Re: [PATCH 5.15.y] block: fix ordering between checking QUEUE_FLAG_QUIESCED request adding Message-ID: <2025042259-gab-earflap-ba40@gregkh> References: <2024120342-monsoon-wildcat-d0a1@gregkh> <20250317072021.22578-1-songmuchun@bytedance.com> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250317072021.22578-1-songmuchun@bytedance.com> On Mon, Mar 17, 2025 at 03:20:21PM +0800, Muchun Song wrote: > Supposing the following scenario. > > CPU0 CPU1 > > blk_mq_insert_request() 1) store > blk_mq_unquiesce_queue() > blk_queue_flag_clear() 3) store > blk_mq_run_hw_queues() > blk_mq_run_hw_queue() > if (!blk_mq_hctx_has_pending()) 4) load > return > blk_mq_run_hw_queue() > if (blk_queue_quiesced()) 2) load > return > blk_mq_sched_dispatch_requests() > > The full memory barrier should be inserted between 1) and 2), as well as > between 3) and 4) to make sure that either CPU0 sees QUEUE_FLAG_QUIESCED > is cleared or CPU1 sees dispatch list or setting of bitmap of software > queue. Otherwise, either CPU will not rerun the hardware queue causing > starvation. > > So the first solution is to 1) add a pair of memory barrier to fix the > problem, another solution is to 2) use hctx->queue->queue_lock to > synchronize QUEUE_FLAG_QUIESCED. Here, we chose 2) to fix it since > memory barrier is not easy to be maintained. > > Fixes: f4560ffe8cec ("blk-mq: use QUEUE_FLAG_QUIESCED to quiesce queue") > Cc: stable@vger.kernel.org > Cc: Muchun Song > Signed-off-by: Muchun Song > Reviewed-by: Ming Lei > Link: https://lore.kernel.org/r/20241014092934.53630-3-songmuchun@bytedance.com > Signed-off-by: Jens Axboe > (cherry picked from commit 6bda857bcbb86fb9d0e54fbef93a093d51172acc) For obvious reasons we can not take a change for an older stable kernel tree, and NOT a newer one. Please resubmit the backports for ALL relevant stable kernel branches. You do not want to upgrade and have a regression. thanks, greg k-h