Re: [PATCH 5/5] block: revert back to synchronous request_queue removal

[Ming Lei]

On Tue, Apr 14, 2020 at 04:19:02AM +0000, Luis Chamberlain wrote:
> Commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression") merged on
> v4.12 moved the work behind blk_release_queue() into a workqueue after a
> splat floated around which indicated some work on blk_release_queue()
> could sleep in blk_exit_rl(). This splat would be possible when a driver
> called blk_put_queue() or blk_cleanup_queue() (which calls blk_put_queue()
> as its final call) from an atomic context.
> 
> blk_put_queue() decrements the refcount for the request_queue
> kobject, and upon reaching 0 blk_release_queue() is called. Although
> blk_exit_rl() is now removed through commit db6d9952356 ("block: remove
> request_list code"), we reserve the right to be able to sleep within
> blk_release_queue() context. If you see no other way and *have* be
> in atomic context when you driver calls the last blk_put_queue()
> you can always just increase your block device's reference count with
> bdgrab() as this can be done in atomic context and the request_queue
> removal would be left to upper layers later. We document this bit of
> tribal knowledge as well now, and adjust kdoc format a bit.
> 
> We revert back to synchronous request_queue removal because asynchronous
> removal creates a regression with expected userspace interaction with
> several drivers. An example is when removing the loopback driver and
> issues ioctl from userspace to do so, upon return and if successful one
> expects the device to be removed. Moving to asynchronous request_queue
> removal could have broken many scripts which relied on the removal to
> have been completed if there was no error.
> 
> Using asynchronous request_queue removal however has helped us find
> other bugs, in the future we can test what could break with this
> arrangement by enabling CONFIG_DEBUG_KOBJECT_RELEASE.
> 
> Cc: Bart Van Assche <bvanassche@xxxxxxx>
> Cc: Omar Sandoval <osandov@xxxxxx>
> Cc: Hannes Reinecke <hare@xxxxxxxx>
> Cc: Nicolai Stange <nstange@xxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: yu kuai <yukuai3@xxxxxxxxxx>
> Suggested-by: Nicolai Stange <nstange@xxxxxxx>
> Fixes: dc9edc44de6c ("block: Fix a blk_exit_rl() regression")
> Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> ---
>  block/blk-core.c       | 19 ++++++++++++++++++-
>  block/blk-sysfs.c      | 38 +++++++++++++++++---------------------
>  include/linux/blkdev.h |  2 --
>  3 files changed, 35 insertions(+), 24 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 5aaae7a1b338..8346c7c59ee6 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -301,6 +301,17 @@ void blk_clear_pm_only(struct request_queue *q)
>  }
>  EXPORT_SYMBOL_GPL(blk_clear_pm_only);
>  
> +/**
> + * blk_put_queue - decrement the request_queue refcount
> + *
> + * Decrements the refcount to the request_queue kobject, when this reaches
> + * 0 we'll have blk_release_queue() called. You should avoid calling
> + * this function in atomic context but if you really have to ensure you
> + * first refcount the block device with bdgrab() / bdput() so that the
> + * last decrement happens in blk_cleanup_queue().
> + *
> + * @q: the request_queue structure to decrement the refcount for
> + */
>  void blk_put_queue(struct request_queue *q)
>  {
>  	kobject_put(&q->kobj);
> @@ -328,10 +339,16 @@ EXPORT_SYMBOL_GPL(blk_set_queue_dying);
>  
>  /**
>   * blk_cleanup_queue - shutdown a request queue
> - * @q: request queue to shutdown
>   *
>   * Mark @q DYING, drain all pending requests, mark @q DEAD, destroy and
>   * put it.  All future requests will be failed immediately with -ENODEV.
> + *
> + * You should not call this function in atomic context. If you need to
> + * refcount a request_queue in atomic context, instead refcount the
> + * block device with bdgrab() / bdput().
> + *
> + * @q: request queue to shutdown
> + *
>   */
>  void blk_cleanup_queue(struct request_queue *q)
>  {
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 0285d67e1e4c..859911191ebc 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -860,22 +860,27 @@ static void blk_exit_queue(struct request_queue *q)
>  	bdi_put(q->backing_dev_info);
>  }
>  
> -
>  /**
> - * __blk_release_queue - release a request queue
> - * @work: pointer to the release_work member of the request queue to be released
> + * blk_release_queue - release a request queue
> + *
> + * This function is called as part of the process when a block device is being
> + * unregistered. Releasing a request queue starts with blk_cleanup_queue(),
> + * which set the appropriate flags and then calls blk_put_queue() as the last
> + * step. blk_put_queue() decrements the reference counter of the request queue
> + * and once the reference counter reaches zero, this function is called to
> + * release all allocated resources of the request queue.
>   *
> - * Description:
> - *     This function is called when a block device is being unregistered. The
> - *     process of releasing a request queue starts with blk_cleanup_queue, which
> - *     set the appropriate flags and then calls blk_put_queue, that decrements
> - *     the reference counter of the request queue. Once the reference counter
> - *     of the request queue reaches zero, blk_release_queue is called to release
> - *     all allocated resources of the request queue.
> + * This function can sleep, and so we must ensure that the very last
> + * blk_put_queue() is never called from atomic context.
> + *
> + * @kobj: pointer to a kobject, who's container is a request_queue
>   */
> -static void __blk_release_queue(struct work_struct *work)
> +static void blk_release_queue(struct kobject *kobj)
>  {
> -	struct request_queue *q = container_of(work, typeof(*q), release_work);
> +	struct request_queue *q =
> +		container_of(kobj, struct request_queue, kobj);
> +
> +	might_sleep();
>  
>  	if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags))
>  		blk_stat_remove_callback(q, q->poll_cb);
> @@ -905,15 +910,6 @@ static void __blk_release_queue(struct work_struct *work)
>  	call_rcu(&q->rcu_head, blk_free_queue_rcu);
>  }
>  
> -static void blk_release_queue(struct kobject *kobj)
> -{
> -	struct request_queue *q =
> -		container_of(kobj, struct request_queue, kobj);
> -
> -	INIT_WORK(&q->release_work, __blk_release_queue);
> -	schedule_work(&q->release_work);
> -}
> -
>  static const struct sysfs_ops queue_sysfs_ops = {
>  	.show	= queue_attr_show,
>  	.store	= queue_attr_store,
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index cc43c8e6516c..81f7ddb1587e 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -582,8 +582,6 @@ struct request_queue {
>  
>  	size_t			cmd_size;
>  
> -	struct work_struct	release_work;
> -
>  #define BLK_MAX_WRITE_HINTS	5
>  	u64			write_hints[BLK_MAX_WRITE_HINTS];
>  };
> -- 
> 2.25.1
> 

Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>

Thanks,
Ming