linux-kernel - Re: [PATCH v4 7/8] mm: vmscan: remove shrinker_rwsem from synchronize

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Date:   Thu, 9 Mar 2023 01:39:52 +0300
From:   Kirill Tkhai <tkhai@...ru>
To:     Qi Zheng <zhengqi.arch@...edance.com>, akpm@...ux-foundation.org,
        hannes@...xchg.org, shakeelb@...gle.com, mhocko@...nel.org,
        roman.gushchin@...ux.dev, muchun.song@...ux.dev, david@...hat.com,
        shy828301@...il.com, rppt@...nel.org
Cc:     sultan@...neltoast.com, dave@...olabs.net,
        penguin-kernel@...ove.SAKURA.ne.jp, paulmck@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 7/8] mm: vmscan: remove shrinker_rwsem from
 synchronize_shrinkers()

On 07.03.2023 09:56, Qi Zheng wrote:
> Now there are no readers of shrinker_rwsem, so
> synchronize_shrinkers() does not need to hold the
> writer of shrinker_rwsem to wait for all running
> shinkers to complete, synchronize_srcu() is enough.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
>  mm/vmscan.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7aaf6f94ac1b..ac7ab4aa344f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -796,15 +796,11 @@ EXPORT_SYMBOL(unregister_shrinker);
>  /**
>   * synchronize_shrinkers - Wait for all running shrinkers to complete.
>   *
> - * This is equivalent to calling unregister_shrink() and register_shrinker(),
> - * but atomically and with less overhead. This is useful to guarantee that all
> - * shrinker invocations have seen an update, before freeing memory, similar to
> - * rcu.
> + * This is useful to guarantee that all shrinker invocations have seen an
> + * update, before freeing memory.
>   */
>  void synchronize_shrinkers(void)
>  {
> -	down_write(&shrinker_rwsem);
> -	up_write(&shrinker_rwsem);
>  	atomic_inc(&shrinker_srcu_generation);
>  	synchronize_srcu(&shrinker_srcu);
>  }

Just curious, callers of synchronize_shrinkers() don't want to have parallel register_shrinker() and unregister_shrink() are completed?
Here we only should wait for parallel shrink_slab(), correct?