Christoph,
On 2018/04/17 8:18, Christoph Hellwig wrote:
> On Mon, Apr 16, 2018 at 06:00:34PM -0700, Bart Van Assche wrote:
>> This patch on itself does not change the behavior of either ioctl.
>> However, this patch is necessary to avoid that these ioctls fail
>> with -EIO if sd_revalidate_disk() is called while these ioctls are
>> in progress because the current zoned block command code temporarily
>> clears data that is needed by these ioctls. See also commit
>> 3ed05a987e0f ("blk-zoned: implement ioctls").
>
> Hmm. I think we need to avoid clearing that data and update it using
> RCU instead. Calling blk_queue_enter before submitting bios is
> something that would make zone reporting very different from any
> other block layer user.
>
With the scsi patches that Bart sent, the data is not cleared anymore until it
is ready to be updated all at once, in between calls to blk_mq_freeze_queue()
and blk_mq_unfreeze_queue(). This prevents the scheduler (deadline and
mq-deadline) from using partially incorrect data since the queue freeze
guarantees no activity on the scheduler side during the data swap.
But for the BIO issuing side, the calls to blk_queue_enter() ensures the same.
This all allows to leave unmodified all the blk-zoned.c helper functions that
use the request queue information (bitmaps and number of zones), without any
need for rcu lock calls. I think this is a simpler approach.
Best regards.
--
Damien Le Moal
Western Digital Research