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

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Message-ID: <CAC5umyi7HyZGb8HX0joMnkJZxsFi16Y9sagEv82hUjhcT0QEXg@mail.gmail.com>
Date:	Wed, 29 Jul 2015 08:37:31 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Wanpeng Li <wanpeng.li@...mail.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>,
	Ming Lei <tom.leiming@...il.com>
Subject: Re: [PATCH v3 7/7] blk-mq: fix deadlock when reading cpu_list

2015-07-28 17:10 GMT+09:00 Wanpeng Li <wanpeng.li@...mail.com>:
>
>
> 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.

Do you suggest that we shouldn't use q->sysfs_lock in
blk_mq_map_swqueue() in this patch?

>>
>> Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
>> Cc: Jens Axboe <axboe@...nel.dk>
>> Cc: 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;
>>
>
>
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