On 4/14/25 6:52 AM, Ming Lei wrote:
> On Fri, Apr 11, 2025 at 12:07:34AM +0530, Nilay Shroff wrote:
>>
>>
>> On 4/10/25 7:00 PM, Ming Lei wrote:
>>> /*
>>> * Use the default elevator settings. If the chosen elevator initialization
>>> * fails, fall back to the "none" elevator (no elevator).
>>> */
>>> -void elevator_init_mq(struct request_queue *q)
>>> +void elevator_set_default(struct request_queue *q)
>>> {
>>> - struct elevator_type *e;
>>> - unsigned int memflags;
>>> + struct elev_change_ctx ctx = { };
>>> int err;
>>>
>>> - WARN_ON_ONCE(blk_queue_registered(q));
>>> -
>>> - if (unlikely(q->elevator))
>>> + if (!queue_is_mq(q))
>>> return;
>>>
>>> - e = elevator_get_default(q);
>>> - if (!e)
>>> + ctx.name = use_default_elevator(q) ? "mq-deadline" : "none";
>>> + if (!q->elevator && !strcmp(ctx.name, "none"))
>>> return;
>>> + err = elevator_change(q, &ctx);
>>> + if (err < 0)
>>> + pr_warn("\"%s\" set elevator failed %d, "
>>> + "falling back to \"none\"\n", ctx.name, err);
>>> +}
>>>
>> If we fail to set the evator to default (mq-deadline) while registering queue,
>> because nr_hw_queue update is simultaneously running then we may end up setting
>> the queue elevator to none and that's not correct. Isn't it?
>
> It still works with none.
>
> I think it isn't one big deal. And if it is really one issue in future, we can
> set one flag in elevator_set_default(), and let blk_mq_update_nr_hw_queues set
> default sched for us.
>
>>
>>> +void elevator_set_none(struct request_queue *q)
>>> +{
>>> + struct elev_change_ctx ctx = {
>>> + .name = "none",
>>> + .uevent = 1,
>>> + };
>>> + int err;
>>>
>>> - blk_mq_unfreeze_queue(q, memflags);
>>> + if (!queue_is_mq(q))
>>> + return;
>>>
>>> - if (err) {
>>> - pr_warn("\"%s\" elevator initialization failed, "
>>> - "falling back to \"none\"\n", e->elevator_name);
>>> - }
>>> + if (!q->elevator)
>>> + return;
>>>
>>> - elevator_put(e);
>>> + err = elevator_change(q, &ctx);
>>> + if (err < 0)
>>> + pr_warn("%s: set none elevator failed %d\n", __func__, err);
>>> }
>>>
>> Here as well if we fail to disable/exit elevator while deleting disk
>> because nr_hw_queue update is simultaneously running then we may
>> leak elevator resource?
>
> When blk_mq_update_nr_hw_queues() observes that queue is dying, it
> forces to change elevator to none, so there isn't elevator leak issue.
>
Yes if we get into blk_mq_update_nr_hw_queues after dying flag is set.
But what if blk_mq_update_nr_hw_queues doesn't see dying flag and starts
running __elevator_change. However later we set dying flag from del_gendisk
and starts running elevator_set_none simultaneously on another cpu?
In this case elevator_set_none would fail to set the elevator to "none" as
blk_mq_update_nr_hw_queues is running on another cpu. Isn't it?
>>
>>> @@ -565,11 +559,7 @@ int __must_check add_disk_fwnode(struct device *parent, struct gendisk *disk,
>>> if (disk->major == BLOCK_EXT_MAJOR)
>>> blk_free_ext_minor(disk->first_minor);
>>> out_exit_elevator:
>>> - if (disk->queue->elevator) {
>>> - mutex_lock(&disk->queue->elevator_lock);
>>> - elevator_exit(disk->queue);
>>> - mutex_unlock(&disk->queue->elevator_lock);
>>> - }
>>> + elevator_set_none(disk->queue);
>> Same comment as above here as well but this is in add_disk code path.
>
> We can avoid it by forcing to change to none in blk_mq_update_nr_hw_queues() for
> !blk_queue_registered()
>
Here as well there's a thin race window possible assuming add_disk fails
after we registered queue. Assuming nr_hw_queue update starts running
and it sees queue is registered however on another cpu add_disk fails
just after registering queue. So in this case still it might be possible
that elevator_set_none might fail to set elevator to "none" just because
nr_hw_queue update is running on another cpu. What do you think?
Thanks,
--Nilay
