Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753255AbbGRQ3R (ORCPT ); Sat, 18 Jul 2015 12:29:17 -0400 Received: from mail-pd0-f175.google.com ([209.85.192.175]:33046 "EHLO mail-pd0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753111AbbGRQ3K (ORCPT ); Sat, 18 Jul 2015 12:29:10 -0400 From: Akinobu Mita To: linux-kernel@vger.kernel.org Cc: Akinobu Mita , Jens Axboe , Ming Lei Subject: [PATCH v3 7/7] blk-mq: fix deadlock when reading cpu_list Date: Sun, 19 Jul 2015 01:28:23 +0900 Message-Id: <1437236903-31617-8-git-send-email-akinobu.mita@gmail.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1437236903-31617-1-git-send-email-akinobu.mita@gmail.com> References: <1437236903-31617-1-git-send-email-akinobu.mita@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2783 Lines: 83 CPU hotplug handling for blk-mq (blk_mq_queue_reinit) acquires all_q_mutex in blk_mq_queue_reinit_notify() and then removes sysfs entries by blk_mq_sysfs_unregister(). Removing sysfs entry needs to be blocked until the active reference of the kernfs_node to be zero. On the other hand, reading blk_mq_hw_sysfs_cpu sysfs entry (e.g. /sys/block/nullb0/mq/0/cpu_list) acquires all_q_mutex in blk_mq_hw_sysfs_cpus_show(). If these happen at the same time, a deadlock can happen. Because one can wait for the active reference to be zero with holding all_q_mutex, and the other tries to acquire all_q_mutex with holding the active reference. The reason that all_q_mutex is acquired in blk_mq_hw_sysfs_cpus_show() is to avoid reading an imcomplete hctx->cpumask. Since reading sysfs entry for blk-mq needs to acquire q->sysfs_lock, we can avoid deadlock and reading an imcomplete hctx->cpumask by protecting q->sysfs_lock while hctx->cpumask is being updated. Signed-off-by: Akinobu Mita Cc: Jens Axboe Cc: Ming Lei --- block/blk-mq-sysfs.c | 4 ---- block/blk-mq.c | 7 +++++++ 2 files changed, 7 insertions(+), 4 deletions(-) diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c index f63b464..e0f71bf 100644 --- a/block/blk-mq-sysfs.c +++ b/block/blk-mq-sysfs.c @@ -218,8 +218,6 @@ static ssize_t blk_mq_hw_sysfs_cpus_show(struct blk_mq_hw_ctx *hctx, char *page) unsigned int i, first = 1; ssize_t ret = 0; - blk_mq_disable_hotplug(); - for_each_cpu(i, hctx->cpumask) { if (first) ret += sprintf(ret + page, "%u", i); @@ -229,8 +227,6 @@ static ssize_t blk_mq_hw_sysfs_cpus_show(struct blk_mq_hw_ctx *hctx, char *page) first = 0; } - blk_mq_enable_hotplug(); - ret += sprintf(ret + page, "\n"); return ret; } diff --git a/block/blk-mq.c b/block/blk-mq.c index b931e38..1a5e7d1 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -1815,6 +1815,11 @@ static void blk_mq_map_swqueue(struct request_queue *q, struct blk_mq_ctx *ctx; struct blk_mq_tag_set *set = q->tag_set; + /* + * Avoid others reading imcomplete hctx->cpumask through sysfs + */ + mutex_lock(&q->sysfs_lock); + queue_for_each_hw_ctx(q, hctx, i) { cpumask_clear(hctx->cpumask); hctx->nr_ctx = 0; @@ -1834,6 +1839,8 @@ static void blk_mq_map_swqueue(struct request_queue *q, hctx->ctxs[hctx->nr_ctx++] = ctx; } + mutex_unlock(&q->sysfs_lock); + queue_for_each_hw_ctx(q, hctx, i) { struct blk_mq_ctxmap *map = &hctx->ctx_map; -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/