On 3/31/19 7:00 PM, Ming Lei wrote:
On Sun, Mar 31, 2019 at 08:27:35AM -0700, Bart Van Assche wrote:
I'm not sure the approach of this patch series is really the direction we
should pursue. There are many block driver that free resources immediately
Please see scsi_run_queue(), and the queue refcount is always held
before run queue.
That's not correct. There is no guarantee that q->q_usage_counter > 0
when scsi_run_queue() is called from inside scsi_requeue_run_queue().
I'd like to avoid having to modify all block drivers that free resources
immediately after blk_cleanup_queue() has returned. Have you considered to
modify blk_mq_run_hw_queues() such that it becomes safe to call that
function while blk_cleanup_queue() is in progress, e.g. by inserting a
percpu_ref_tryget_live(&q->q_usage_counter) /
percpu_ref_put(&q->q_usage_counter) pair?
It can't work because blk_mq_run_hw_queues may happen after
percpu_ref_exit() is done.
However, if we move percpu_ref_exit() into queue's release handler, we
don't need to grab q->q_usage_counter any more in blk_mq_run_hw_queues(),
and we still have to free hw queue resources in queue's release handler,
that is exactly what this patchset is doing.
In short, getting q->q_usage_counter doesn't make a difference on this
issue.
percpu_ref_tryget_live() fails if a per-cpu counter is in the "dead"
state. percpu_ref_kill() changes the state of a per-cpu counter to the
"dead" state. blk_freeze_queue_start() calls percpu_ref_kill().
blk_cleanup_queue() already calls blk_set_queue_dying() and that last
function calls blk_freeze_queue_start(). So I think that what you wrote
is not correct and that inserting a
percpu_ref_tryget_live()/percpu_ref_put() pair in blk_mq_run_hw_queues()
or blk_mq_run_hw_queue() would make a difference and also that moving
the percpu_ref_exit() call into blk_release_queue() makes sense.
Bart.
