Re: [PATCH v3 7/7] blk-mq: fix deadlock when reading cpu_list

[Wanpeng Li]

On 7/19/15 12:28 AM, Akinobu Mita 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.

Dump ctx attrs will be excluded with map software to hardware queue 
operations which makes people confusing after your patch.

Regards,
Wanpeng Li

> Signed-off-by: Akinobu Mita <akinobu.mita@xxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Ming Lei <tom.leiming@xxxxxxxxx>
> ---
>   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;
>   

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