Re: [PATCH] block: use trylock to avoid lockdep circular dependency in sysfs

[Jens Axboe <axboe@kernel.dk>]

On 3/4/26 8:15 PM, Ming Lei wrote:
> Use trylock instead of blocking lock acquisition for update_nr_hwq_lock
> in queue_requests_store() and elv_iosched_store() to avoid circular lock
> dependency with kernfs active reference during concurrent disk deletion:
> 
>   update_nr_hwq_lock -> kn->active (via del_gendisk -> kobject_del)
>   kn->active -> update_nr_hwq_lock (via sysfs write path)
> 
> Return -EBUSY when the lock is not immediately available.

This does mean that scripts can get spurious -EBUSY they now need to
handle. Which probably isn't a big real world problem. While it isn't a
hard thing to fix up in a script, it can potentially break existing
setups.

Not sure I see a good way around that however, without queueing these
to a clean context and having the store helpers wait on eg work item
being processed and completed.

Hopefully this will never be a thing and we'll never need to discuss
this issue, I'll get this one applied for now.

-- 
Jens Axboe