Re: [PATCH 2/3] block: introduce alloc_sched_data and free_sched_data elevator methods

[Ming Lei <ming.lei@redhat.com>]

On Thu, Oct 16, 2025 at 11:00:48AM +0530, Nilay Shroff wrote:
> The recent lockdep splat [1] highlights a potential deadlock risk
> involving ->elevator_lock and ->freeze_lock dependencies on -pcpu_alloc_
> mutex. The trace shows that the issue occurs when the Kyber scheduler
> allocates dynamic memory for its elevator data during initialization.
> 
> To address this, introduce two new elevator operation callbacks:
> ->alloc_sched_data and ->free_sched_data.

This way looks good.

> 
> When an elevator implements these methods, they are invoked during
> scheduler switch before acquiring ->freeze_lock and ->elevator_lock.
> This allows safe allocation and deallocation of per-elevator data

This per-elevator data should be very similar with `struct elevator_tags`
from block layer viewpoint: both have same lifetime, and follow same
allocation constraint(per-cpu lock).

Can we abstract elevator data structure to cover both? Then I guess the
code should be more readable & maintainable, what do you think of this way?

One easiest way could be to add 'void *data' into `struct elevator_tags`,
just the naming of `elevator_tags` is not generic enough, but might not
a big deal.


Thanks,
Ming