Re: [PATCHv4 5/5] block: define alloc_sched_data and free_sched_data methods for kyber

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

On Mon, Nov 10, 2025 at 01:44:52PM +0530, Nilay Shroff wrote:
> Currently, the Kyber elevator allocates its private data dynamically in
> ->init_sched and frees it in ->exit_sched. However, since ->init_sched
> is invoked during elevator switch after acquiring both ->freeze_lock and
> ->elevator_lock, it may trigger the lockdep splat [1] due to dependency
> on pcpu_alloc_mutex.
> 
> To resolve this, move the elevator data allocation and deallocation
> logic from ->init_sched and ->exit_sched into the newly introduced
> ->alloc_sched_data and ->free_sched_data methods. These callbacks are
> invoked before acquiring ->freeze_lock and ->elevator_lock, ensuring
> that memory allocation happens safely without introducing additional
> locking dependencies.
> 
> This change breaks the dependency chain involving pcpu_alloc_mutex and
> prevents the reported lockdep warning.
> 
> [1] https://lore.kernel.org/all/CAGVVp+VNW4M-5DZMNoADp6o2VKFhi7KxWpTDkcnVyjO0=-D5+A@mail.gmail.com/
> 
> Reported-by: Changhui Zhong <czhong@redhat.com>
> Reported-by: Yi Zhang <yi.zhang@redhat.com>
> Closes: https://lore.kernel.org/all/CAGVVp+VNW4M-5DZMNoADp6o2VKFhi7KxWpTDkcnVyjO0=-D5+A@mail.gmail.com/
> Tested-by: Yi Zhang <yi.zhang@redhat.com>
> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming