linux-kernel - Re: [PATCH v3 7/7] blk-mq: fix deadlock when reading cpu

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Date:	Sun, 19 Jul 2015 20:30:59 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v3 7/7] blk-mq: fix deadlock when reading cpu_list

On Sun, Jul 19, 2015 at 12:28 AM, Akinobu Mita <akinobu.mita@...il.com> wrote:
> 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 <akinobu.mita@...il.com>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Ming Lei <tom.leiming@...il.com>

Reviewed-by: Ming Lei <tom.leiming@...il.com>

> ---
>  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
>



-- 
Ming Lei
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