linux-kernel - Re: [PATCH v2 09/12] md/raid5: Keep a reference to last stripe

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Message-ID: <YmD3Ao1lLH3LakZx@infradead.org>
Date:   Wed, 20 Apr 2022 23:17:38 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        Song Liu <song@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Stephen Bates <sbates@...thlin.com>,
        Martin Oliveira <Martin.Oliveira@...eticom.com>,
        David Sloan <David.Sloan@...eticom.com>
Subject: Re: [PATCH v2 09/12] md/raid5: Keep a reference to last stripe_head
 for batch

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@....de>