Re: [PATCH v2 09/12] md/raid5: Keep a reference to last stripe_head for batch

[Christoph Hellwig]

On Wed, Apr 20, 2022 at 01:54:22PM -0600, Logan Gunthorpe wrote:
> When batching, every stripe head has to find the previous stripe head to
> add to the batch list. This involves taking the hash lock which is
> highly contended during IO.
> 
> Instead of finding the previous stripe_head each time, store a
> reference to the previous stripe_head in a pointer so that it doesn't
> require taking the contended lock another time.
> 
> The reference to the previous stripe must be released before scheduling
> and waiting for work to get done. Otherwise, it can hold up
> raid5_activate_delayed() and deadlock.

Looks good:

Reviewed-by: Christoph Hellwig <hch@xxxxxx>