From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757523AbaLIP6k (ORCPT ); Tue, 9 Dec 2014 10:58:40 -0500 Received: from xavier.telenet-ops.be ([195.130.132.52]:36196 "EHLO xavier.telenet-ops.be" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756816AbaLIP6j (ORCPT ); Tue, 9 Dec 2014 10:58:39 -0500 Message-ID: <54871C2B.8070605@acm.org> Date: Tue, 09 Dec 2014 16:58:35 +0100 From: Bart Van Assche User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 MIME-Version: 1.0 To: Jens Axboe CC: Christoph Hellwig , Robert Elliott , Ming Lei , Alexander Gordeev , linux-kernel Subject: [PATCH 3/6] blk-mq: Fix a race between bt_clear_tag() and bt_get() References: <54871BD0.8020305@acm.org> In-Reply-To: <54871BD0.8020305@acm.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org What we need is the following two guarantees: * Any thread that observes the effect of the test_and_set_bit() by __bt_get_word() also observes the preceding addition of 'current' to the appropriate wait list. This is guaranteed by the semantics of the spin_unlock() operation performed by prepare_and_wait(). Hence the conversion of test_and_set_bit_lock() into test_and_set_bit(). * The wait lists are examined by bt_clear() after the tag bit has been cleared. clear_bit_unlock() guarantees that any thread that observes that the bit has been cleared also observes the store operations preceding clear_bit_unlock(). However, clear_bit_unlock() does not prevent that the wait lists are examined before that the tag bit is cleared. Hence the addition of a memory barrier between clear_bit() and the wait list examination. Signed-off-by: Bart Van Assche Cc: Christoph Hellwig Cc: Robert Elliott Cc: Ming Lei Cc: Alexander Gordeev Cc: # v3.13+ --- block/blk-mq-tag.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c index 5a3db28..70d9f9e 100644 --- a/block/blk-mq-tag.c +++ b/block/blk-mq-tag.c @@ -158,7 +158,7 @@ restart: return -1; } last_tag = tag + 1; - } while (test_and_set_bit_lock(tag, &bm->word)); + } while (test_and_set_bit(tag, &bm->word)); return tag; } @@ -342,11 +342,10 @@ static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag) struct bt_wait_state *bs; int wait_cnt; - /* - * The unlock memory barrier need to order access to req in free - * path and clearing tag bit - */ - clear_bit_unlock(TAG_TO_BIT(bt, tag), &bt->map[index].word); + clear_bit(TAG_TO_BIT(bt, tag), &bt->map[index].word); + + /* Ensure that the wait list checks occur after clear_bit(). */ + smp_mb(); bs = bt_wake_ptr(bt); if (!bs) -- 2.1.2