Re: [for-4.16 PATCH v3 2/3] block: allow gendisk's request_queue registration to be deferred

Hannes Reinecke
2018-01-11T07:56:06Z

On 01/11/2018 03:12 AM, Mike Snitzer wrote:
> Since I can remember DM has forced the block layer to allow the
> allocation and initialization of the request_queue to be distinct
> operations. Reason for this is block/genhd.c:add_disk() has requires
> that the request_queue (and associated bdi) be tied to the gendisk
> before add_disk() is called -- because add_disk() also deals with
> exposing the request_queue via blk_register_queue().
>
> DM's dynamic creation of arbitrary device types (and associated
> request_queue types) requires the DM device's gendisk be available so
> that DM table loads can establish a master/slave relationship with
> subordinate devices that are referenced by loaded DM tables -- using
> bd_link_disk_holder(). But until these DM tables, and their associated
> subordinate devices, are known DM cannot know what type of request_queue
> it needs -- nor what its queue_limits should be.
>
> This chicken and egg scenario has created all manner of problems for DM
> and, at times, the block layer.
>
> Summary of changes:
>
> - Add QUEUE_FLAG_DEFER_REG that a block driver can set to defer the
> required blk_register_queue() until the block driver's request_queue
> is fully initialized.
>
> - Return early from blk_unregister_queue() if QUEUE_FLAG_REGISTERED
> is not set. It won't be set if a request_queue with
> QUEUE_FLAG_DEFER_REG set never called blk_register_queue() -- as can
> happen if a driver encounters an error and must del_gendisk() before
> calling blk_register_queue().
>
> - Export blk_register_queue().
>
> These changes allow DM to use device_add_disk() to anchor its gendisk as
> the "master" for master/slave relationships DM must establish with
> subordinate devices referenced in DM tables that get loaded. Once all
> "slave" devices for a DM device are known a request_queue can be
> properly initialized and then advertised via sysfs -- important
> improvement being that no request_queue resource initialization is
> missed -- before these changes DM was known to be missing blk-mq debugfs
> and proper block throttle initialization.
>
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> ---
> block/blk-sysfs.c | 4 ++++
> block/genhd.c | 4 +++-
> include/linux/blkdev.h | 1 +
> 3 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 870484eaed1f..2395122875b4 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -921,6 +921,7 @@ int blk_register_queue(struct gendisk *disk)
> mutex_unlock(&q->sysfs_lock);
> return ret;
> }
> +EXPORT_SYMBOL_GPL(blk_register_queue);
>
> void blk_unregister_queue(struct gendisk *disk)
> {
> @@ -929,6 +930,9 @@ void blk_unregister_queue(struct gendisk *disk)
> if (WARN_ON(!q))
> return;
>
> + if (!test_bit(QUEUE_FLAG_REGISTERED, &disk->queue->queue_flags))
> + return;
> +
> mutex_lock(&q->sysfs_lock);
> queue_flag_clear_unlocked(QUEUE_FLAG_REGISTERED, q);
> mutex_unlock(&q->sysfs_lock);
> diff --git a/block/genhd.c b/block/genhd.c
> index 00620e01e043..3912a82ecc4f 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -682,7 +682,6 @@ void device_add_disk(struct device *parent, struct gendisk *disk)
> exact_match, exact_lock, disk);
> }
> register_disk(parent, disk);
> - blk_register_queue(disk);
>
> /*
> * Take an extra ref on queue which will be put on disk_release()
> @@ -692,6 +691,9 @@ void device_add_disk(struct device *parent, struct gendisk *disk)
>
> disk_add_events(disk);
> blk_integrity_add(disk);
> +
> + if (!test_bit(QUEUE_FLAG_DEFER_REG, &disk->queue->queue_flags))
> + blk_register_queue(disk);
> }
> EXPORT_SYMBOL(device_add_disk);
>
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 71a9371c8182..84d144c7b025 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -681,6 +681,7 @@ struct request_queue {
> #define QUEUE_FLAG_SCSI_PASSTHROUGH 27 /* queue supports SCSI commands */
> #define QUEUE_FLAG_QUIESCED 28 /* queue has been quiesced */
> #define QUEUE_FLAG_PREEMPT_ONLY 29 /* only process REQ_PREEMPT requests */
> +#define QUEUE_FLAG_DEFER_REG 30 /* defer registering queue to a disk */
>
> #define QUEUE_FLAG_DEFAULT ((1 << QUEUE_FLAG_IO_STAT) | \
> (1 << QUEUE_FLAG_SAME_COMP) | \
>
Thinking of this a bit more, wouldn't it be better to modify add_disk()
(or, rather, device_add_disk()) to handle this case?
You already moved the call to 'blk_register_queue()' to the end of
device_add_disk(), so it would be trivial to make device_add_disk() a
wrapper function like

void device_add_disk(struct device *parent, struct gendisk *disk) {
blk_add_disk(parent, disk);
blk_register_queue(disk);
}

and then call blk_add_disk() from the dm-core.
That would save us the magic 'you have to set this flag before
registering' dance in dm.c...

Cheers,

Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
hare@suse.de +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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