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Re: [PATCH v4 6/7] blk-mq: fix freeze queue race

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Hi Tejun,

2015-09-27 2:32 GMT+09:00 Tejun Heo <tj@kernel.org>:
> Hello,
>
> On Sun, Sep 27, 2015 at 02:09:24AM +0900, Akinobu Mita wrote:
>> @@ -420,7 +420,9 @@ static void blk_mq_sysfs_init(struct request_queue *q)
>>  /* see blk_register_queue() */
>>  void blk_mq_finish_init(struct request_queue *q)
>>  {
>> +     mutex_lock(&q->mq_freeze_lock);
>>       percpu_ref_switch_to_percpu(&q->mq_usage_counter);
>> +     mutex_unlock(&q->mq_freeze_lock);
>
> This looks weird to me.  What can it race against at this point?

The possible scenario is described in commit log (1. ~ 7.).  In summary,
blk_mq_finish_init() and blk_mq_freeze_queue_start() can be executed
at the same time, so this is required to serialize the execution of
percpu_ref_switch_to_percpu() by blk_mq_finish_init() and
percpu_ref_kill() by blk_mq_freeze_queue_start().

>> @@ -115,11 +115,15 @@ void blk_mq_freeze_queue_start(struct request_queue *q)
>>  {
>>       int freeze_depth;
>>
>> +     mutex_lock(&q->mq_freeze_lock);
>> +
>>       freeze_depth = atomic_inc_return(&q->mq_freeze_depth);
>
> It doesn't have to be an atomic anymore, right?

Yes, you are right.  I would like to make it in another patch in order to
simplify each change.
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[PATCH v4 0/7] blk-mq: fix race conditions on cpu hotplug handling Akinobu Mita <akinobu.mita@gmail.com> - 2015-09-26 19:10 +0200
  [PATCH v4 4/7] blk-mq: fix q->mq_usage_counter access race Akinobu Mita <akinobu.mita@gmail.com> - 2015-09-26 19:20 +0200
  [PATCH v4 5/7] blk-mq: avoid inserting requests before establishing new mapping Akinobu Mita <akinobu.mita@gmail.com> - 2015-09-26 19:20 +0200
    Re: [PATCH v4 5/7] blk-mq: avoid inserting requests before  establishing new mapping Christoph Hellwig <hch@lst.de> - 2015-09-29 09:00 +0200
  [PATCH v4 2/7] blk-mq: fix sysfs registration/unregistration race Akinobu Mita <akinobu.mita@gmail.com> - 2015-09-26 19:20 +0200
  [PATCH v4 1/7] blk-mq: avoid setting hctx->tags->cpumask before allocation Akinobu Mita <akinobu.mita@gmail.com> - 2015-09-26 19:20 +0200
    Re: [PATCH v4 1/7] blk-mq: avoid setting hctx->tags->cpumask  before allocation Christoph Hellwig <hch@lst.de> - 2015-09-26 20:20 +0200
  [PATCH v4 7/7] blk-mq: fix deadlock when reading cpu_list Akinobu Mita <akinobu.mita@gmail.com> - 2015-09-26 19:20 +0200
  [PATCH v4 6/7] blk-mq: fix freeze queue race Akinobu Mita <akinobu.mita@gmail.com> - 2015-09-26 19:20 +0200
    Re: [PATCH v4 6/7] blk-mq: fix freeze queue race Tejun Heo <tj@kernel.org> - 2015-09-26 19:40 +0200
      Re: [PATCH v4 6/7] blk-mq: fix freeze queue race Akinobu Mita <akinobu.mita@gmail.com> - 2015-09-27 15:10 +0200
        Re: [PATCH v4 6/7] blk-mq: fix freeze queue race Tejun Heo <tj@kernel.org> - 2015-09-28 16:50 +0200
          Re: [PATCH v4 6/7] blk-mq: fix freeze queue race Christoph Hellwig <hch@lst.de> - 2015-09-29 09:00 +0200
          Re: [PATCH v4 6/7] blk-mq: fix freeze queue race Tejun Heo <tj@kernel.org> - 2015-09-29 17:10 +0200
            Re: [PATCH v4 6/7] blk-mq: fix freeze queue race Tejun Heo <tj@kernel.org> - 2015-09-30 00:00 +0200
              Re: [PATCH v4 6/7] blk-mq: fix freeze queue race Akinobu Mita <akinobu.mita@gmail.com> - 2015-09-30 12:40 +0200
          Re: [PATCH v4 6/7] blk-mq: fix freeze queue race Jens Axboe <axboe@kernel.dk> - 2015-09-29 17:10 +0200
  [PATCH v4 3/7] blk-mq: Fix use after of free q->mq_map Akinobu Mita <akinobu.mita@gmail.com> - 2015-09-26 19:20 +0200
    Re: [PATCH v4 3/7] blk-mq: Fix use after of free q->mq_map Christoph Hellwig <hch@lst.de> - 2015-09-29 09:00 +0200
  Re: [PATCH v4 0/7] blk-mq: fix race conditions on cpu hotplug  handling Jens Axboe <axboe@kernel.dk> - 2015-09-29 19:40 +0200

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