Re: [PATCH v2 1/6] blk-mq: fix sysfs registration/unregistration race

[Ming Lei]

On Thu, Jul 9, 2015 at 10:25 PM, Akinobu Mita <akinobu.mita@xxxxxxxxx> wrote:
> Hi Ming,
>
> 2015-07-08 20:48 GMT+09:00 Ming Lei <tom.leiming@xxxxxxxxx>:
>> On Thu, Jul 2, 2015 at 10:29 PM, Akinobu Mita <akinobu.mita@xxxxxxxxx> wrote:
>>> There is a race between cpu hotplug handling and adding/deleting
>>> gendisk for blk-mq, where both are trying to register and unregister
>>> the same sysfs entries.
>>>
>>> CPU hotplug handling for blk-mq (blk_mq_queue_reinit) does unregister
>>> and register sysfs entries of hctx for all request queues in
>>> all_q_list.  On the other hand, these entries for a request queue may
>>> have already been unregistered or in the middle of registration when
>>> CPU hotplug event occurs.  Because those sysfs entries are registered
>>> by blk_mq_register_disk() after the request queue is added to
>>> all_q_list, and similarily the request queue is deleted from
>>> all_q_list after those are unregistered by blk_mq_unregister_disk().
>>>
>>> So we need proper mutual exclusion for those registration and
>>> unregistration.
>>
>> But that may not be enough, and you miss another point in which
>> blk_mq_sysfs_register() has to be called after the remapping is built,
>> so the introduced lock should have been held in blk_mq_queue_reinit().
>
> OK.
>
> Although q->mq_sysfs_lock is held in blk_mq_sysfs_register() by
> this patch, it should be held in blk_mq_queue_reinit() as you
> suggested.  Since hctx->nr_ctx is reset temporarily in
> blk_mq_map_swqueue() while cpu hotplug callback is called, so we
> should avoid blk_mq_register_disk() seeing the temporary
> hctx->nr_ctx value (i.e. zero) at the same time by holding
> q->mq_sysfs_lock in blk_mq_queue_reinit().
>
>>> Signed-off-by: Akinobu Mita <akinobu.mita@xxxxxxxxx>
>>> Cc: Jens Axboe <axboe@xxxxxxxxx>
>>> Cc: Ming Lei <tom.leiming@xxxxxxxxx>
>>> ---
>>>  block/blk-core.c       |  1 +
>>>  block/blk-mq-sysfs.c   | 44 ++++++++++++++++++++++++++++++++++----------
>>>  include/linux/blkdev.h |  3 +++
>>>  3 files changed, 38 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/block/blk-core.c b/block/blk-core.c
>>> index 82819e6..bbf67cd 100644
>>> --- a/block/blk-core.c
>>> +++ b/block/blk-core.c
>>> @@ -687,6 +687,7 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
>>>         __set_bit(QUEUE_FLAG_BYPASS, &q->queue_flags);
>>>
>>>         init_waitqueue_head(&q->mq_freeze_wq);
>>> +       mutex_init(&q->mq_sysfs_lock);
>>
>> The above should be put into blk_mq_init_allocated_queue().
>
> OK, make sense.
>
>>>
>>>         if (blkcg_init_queue(q))
>>>                 goto fail_bdi;
>>> diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
>>> index b79685e..79a3e8d 100644
>>> --- a/block/blk-mq-sysfs.c
>>> +++ b/block/blk-mq-sysfs.c
>>> @@ -332,13 +332,15 @@ static void blk_mq_unregister_hctx(struct blk_mq_hw_ctx *hctx)
>>>         struct blk_mq_ctx *ctx;
>>>         int i;
>>>
>>> -       if (!hctx->nr_ctx || !(hctx->flags & BLK_MQ_F_SYSFS_UP))
>>> +       if (!(hctx->flags & BLK_MQ_F_SYSFS_UP))
>>>                 return;
>>
>> It isn't needed to introduce above change.
>>
>>>
>>>         hctx_for_each_ctx(hctx, ctx, i)
>>>                 kobject_del(&ctx->kobj);
>>>
>>>         kobject_del(&hctx->kobj);
>>> +
>>> +       hctx->flags &= ~BLK_MQ_F_SYSFS_UP;
>>
>> I guess you misunderstand the flag, which just means the syfs stuff
>> for the hcts has been setup, and you needn't clear it.
>
> I've changed the meaning of the flag which should have been
> mentioned and also introduced q->mq_sysfs_init_done.  So I admit
> that the code has become much complicated.
>
> But after consideration, I think we can only need q->mq_sysfs_init_done
> and BLK_MQ_F_SYSFS_UP can be removed.  Please see below.
>
>>>  }
>>>
>>>  static int blk_mq_register_hctx(struct blk_mq_hw_ctx *hctx)
>>> @@ -347,7 +349,7 @@ static int blk_mq_register_hctx(struct blk_mq_hw_ctx *hctx)
>>>         struct blk_mq_ctx *ctx;
>>>         int i, ret;
>>>
>>> -       if (!hctx->nr_ctx || !(hctx->flags & BLK_MQ_F_SYSFS_UP))
>>> +       if (!hctx->nr_ctx)
>>>                 return 0;
>>>
>>>         ret = kobject_add(&hctx->kobj, &q->mq_kobj, "%u", hctx->queue_num);
>>> @@ -357,10 +359,12 @@ static int blk_mq_register_hctx(struct blk_mq_hw_ctx *hctx)
>>>         hctx_for_each_ctx(hctx, ctx, i) {
>>>                 ret = kobject_add(&ctx->kobj, &hctx->kobj, "cpu%u", ctx->cpu);
>>>                 if (ret)
>>> -                       break;
>>> +                       return ret;
>>>         }
>>>
>>> -       return ret;
>>> +       hctx->flags |= BLK_MQ_F_SYSFS_UP;
>>
>> Same with above.
>>
>>> +
>>> +       return 0;
>>>  }
>>>
>>>  void blk_mq_unregister_disk(struct gendisk *disk)
>>> @@ -370,6 +374,8 @@ void blk_mq_unregister_disk(struct gendisk *disk)
>>>         struct blk_mq_ctx *ctx;
>>>         int i, j;
>>>
>>> +       mutex_lock(&q->mq_sysfs_lock);
>>> +
>>>         queue_for_each_hw_ctx(q, hctx, i) {
>>>                 blk_mq_unregister_hctx(hctx);
>>>
>>> @@ -384,6 +390,9 @@ void blk_mq_unregister_disk(struct gendisk *disk)
>>>         kobject_put(&q->mq_kobj);
>>>
>>>         kobject_put(&disk_to_dev(disk)->kobj);
>>> +
>>> +       q->mq_sysfs_init_done = false;
>>
>> The flag isn't needed, but you should hold q->mq_sysfs_lock
>> inside blk_mq_queue_reinit().
>
> The reason for q->mq_sysfs_init_done is to avoid the following
> scenario.
>
> While the request queue is added to 'all_q_list' (*),
> blk_mq_queue_reinit() can be called for the queue anytime by CPU
> hotplug callback.  But blk_mq_sysfs_unregister (-) and
> blk_mq_sysfs_register (+) in blk_mq_queue_reinit must not be
> called before blk_mq_register_disk (++) is finished and after
> blk_mq_unregister_disk (--).  Because '/sys/block/nullb*/mq/' is
> not exists.

Yes.

>
> BLK_MQ_F_SYSFS_UP flag in hctx->flags fixes this issue partially now.
> In order to fix it completely, we just need q->mq_sysfs_init_done
> instead of per hctx flag with appropriate locking.

One flag should be OK, but keeping both just makes things
complicated, and looks mq_sysfs_init_done is better.

>
> null_add_dev
>     --> blk_mq_init_queue
>         --> blk_mq_init_allocated_queue
>             --> add to 'all_q_list' (*)
>     --> add_disk
>         --> blk_register_queue
>             --> blk_mq_register_disk (++)
>
> null_del_dev
>     --> del_gendisk
>         --> blk_unregister_queue
>             --> blk_mq_unregister_disk (--)
>     --> blk_cleanup_queue
>         --> blk_mq_free_queue
>             --> del from 'all_q_list' (*)
>
> blk_mq_queue_reinit
>     --> blk_mq_sysfs_unregister (-)
>     --> blk_mq_sysfs_register (+)



-- 
Ming Lei
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