Re: [RFC PATCH v2 2/2] blk-mq: Lockout tagset iter when freeing rqs

[Bart Van Assche <bvanassche@acm.org>]

On 1/4/21 7:33 AM, John Garry wrote:
> On 23/12/2020 15:47, Bart Van Assche wrote:
>> I propose to change the order in which blk_mq_sched_free_requests(q) and
>> blk_mq_debugfs_unregister(q) are called. Today blk_mq_sched_free_requests(q)
>> is called by blk_cleanup_queue() before blk_put_queue() is called.
>> blk_put_queue() calls blk_release_queue() if the last reference is dropped.
>> blk_release_queue() calls blk_mq_debugfs_unregister(). I prefer removing the
>> debugfs attributes earlier over modifying the tag iteration functions
>> because I think removing the debugfs attributes earlier is less risky.
> 
> But don't we already have this following path to remove the per-hctx debugfs
> dir earlier than blk_mq_sched_free_requests() or blk_release_queue():
> 
> blk_cleanup_queue() -> blk_mq_exit_queue() -> blk_mq_exit_hw_queues() ->
> blk_mq_debugfs_unregister_hctx() ->
> blk_mq_debugfs_unregister_hctx(hctx->debugfs_dir)
> 
> Having said that, I am not sure how this is related directly to the problem
> I mentioned. In that problem, above, we trigger the
> blk_mq_tagset_busy_iter() from the SCSI host sysfs file, and the
> use-after-free comes about from disabling the elevator (and freeing the
> sched requests) in parallel.

Hi John,

Right, what I proposed is unrelated to the use-after-free triggered by
disabling I/O scheduling.

Regarding the races triggered by disabling I/O scheduling: can these be
fixed by quiescing all request queues associated with a tag set before
changing the I/O scheduler instead of only the request queue for which the
I/O scheduler is changed? I think we already do this before updating the
number of hardware queues.

Thanks,

Bart.