From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 6F2101D9357 for ; Wed, 4 Sep 2024 12:56:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725454598; cv=none; b=RZWLk/ojCgLJPcqduwTmvBTS0xSPZdaaYLv/B5wffRi9Cwxr0rPWv2zDyR7ywi1aNfh1aXVnMUrgoyDwQRjRJEegdzb9vp9nafV2626Po4yWl+Zck+87PJEIuuUUd4j4JtBB/ItobzmEjX17A0Q1bpXc033Rf4rdOXKhwTSOKWU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725454598; c=relaxed/simple; bh=w12zITrlcS7mKMPlkOl2ZGejq1Eo5bn874xDYkUxjBU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=aXUpA5K3loNeRrH3xEA/jdgrp8muID4Lha/aCRBploUcAr8JENx0mo570voVl3/Nmic7fDucsqzALOG8qRXPUXKRUwj1ZQqM75Gm0KL65/O/YeEG+4uYqbqAXSUmQLzs+RM7M6KffHVJuBWh+Madh3S4nKmU9z9D569lgQc2WAs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=HqSZ6Csp; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="HqSZ6Csp" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1725454594; 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: in-reply-to:in-reply-to:references:references; bh=1Y6zLyuFowAQXUpe4yq3nH5ym9sfNZkhp6uZ2OTgfrU=; b=HqSZ6CspTcwqSeSyQpMD3doc0PdWW6O2PJuv6PGZm4AV5/5isqRkQejN21usPgAw6ka7vW 37vOszGYH3PWVCvN732KhfmK7dZB/1jaO5uCAJEhPLlV3lIMYJOxil0rBMVKi6LKcVmX35 Te3CAdk76GvHmqeA8N7eOJ5pnidgrdw= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-2-3Cjfp9NyMZSq6G0H6OWfpw-1; Wed, 04 Sep 2024 08:56:31 -0400 X-MC-Unique: 3Cjfp9NyMZSq6G0H6OWfpw-1 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id AEDDE1955DD2; Wed, 4 Sep 2024 12:56:29 +0000 (UTC) Received: from fedora (unknown [10.72.116.59]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 3F0E5195605A; Wed, 4 Sep 2024 12:56:23 +0000 (UTC) Date: Wed, 4 Sep 2024 20:56:18 +0800 From: Ming Lei To: Muchun Song Cc: axboe@kernel.dk, yukuai1@huaweicloud.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, muchun.song@linux.dev, stable@vger.kernel.org Subject: Re: [PATCH v2 2/3] block: fix ordering between checking QUEUE_FLAG_QUIESCED and adding requests Message-ID: References: <20240903081653.65613-1-songmuchun@bytedance.com> <20240903081653.65613-3-songmuchun@bytedance.com> Precedence: bulk X-Mailing-List: linux-block@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: <20240903081653.65613-3-songmuchun@bytedance.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 On Tue, Sep 03, 2024 at 04:16:52PM +0800, Muchun Song wrote: > Supposing the following scenario. > > CPU0 CPU1 > > blk_mq_insert_request() 1) store blk_mq_unquiesce_queue() > blk_mq_run_hw_queue() blk_queue_flag_clear(QUEUE_FLAG_QUIESCED) 3) store > if (blk_queue_quiesced()) 2) load blk_mq_run_hw_queues() > return blk_mq_run_hw_queue() > blk_mq_sched_dispatch_requests() if (!blk_mq_hctx_has_pending()) 4) load > return > > 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 re-run 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: f4560ffe8cec1 ("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 thanks, Ming