Messages in this thread | | | Subject | Re: [RFC PATCH v3 2/3] blk-mq: Freeze and quiesce all queues for tagset in elevator_exit() | From | John Garry <> | Date | Mon, 8 Mar 2021 10:50:55 +0000 |
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On 06/03/2021 04:32, Bart Van Assche wrote: > On 3/5/21 7:14 AM, John Garry wrote: >> diff --git a/block/blk.h b/block/blk.h >> index 3b53e44b967e..1a948bfd91e4 100644 >> --- a/block/blk.h >> +++ b/block/blk.h >> @@ -201,10 +201,29 @@ void elv_unregister_queue(struct request_queue *q); >> static inline void elevator_exit(struct request_queue *q, >> struct elevator_queue *e) >> { >> + struct blk_mq_tag_set *set = q->tag_set; >> + struct request_queue *tmp; >> + >> lockdep_assert_held(&q->sysfs_lock); >> >> + mutex_lock(&set->tag_list_lock); >> + list_for_each_entry(tmp, &set->tag_list, tag_set_list) { >> + if (tmp == q) >> + continue; >> + blk_mq_freeze_queue(tmp); >> + blk_mq_quiesce_queue(tmp); >> + } >> + >> blk_mq_sched_free_requests(q); >> __elevator_exit(q, e); >> + >> + list_for_each_entry(tmp, &set->tag_list, tag_set_list) { >> + if (tmp == q) >> + continue; >> + blk_mq_unquiesce_queue(tmp); >> + blk_mq_unfreeze_queue(tmp); >> + } >> + mutex_unlock(&set->tag_list_lock); >> }
Hi Bart,
> This patch introduces nesting of tag_list_lock inside sysfs_lock. The > latter is per request queue while the former can be shared across > multiple request queues. Has it been analyzed whether this is safe?
Firstly - ignoring implementation details for a moment - this patch is to ensure that the concept is consistent with your suggestion and whether it is sound.
As for nested locking, I can analyze more, but I did assume that we don't care about locking-out sysfs intervention during this time. And it seems pretty difficult to avoid nesting the locks.
And further to this, I see that https://lore.kernel.org/linux-block/3aa5407c-0800-2482-597b-4264781a7eac@grimberg.me/T/#mc3e3175642660578c0ae2a6c32185b1e34ec4b8a has a new interface for tagset quiesce, which could make this process more efficient.
Please let me know further thoughts.
Thanks, John
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