Re: [PATCH v2 2/3] writeback: drop now-unnecessary rcu_barrier() in cgroup_writeback_umount()

[Jan Kara]

On Sun 17-05-26 22:21:31, Baokun Li wrote:
> Commit e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when
> switching inodes") replaced the queue_rcu_work() based scheduling of
> inode wb switches with a plain queue_work().  Since then no switcher
> goes through call_rcu(), so rcu_barrier() in cgroup_writeback_umount()
> has no work to wait for and is effectively a no-op.
> 
> Fixes: e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes")
> Signed-off-by: Baokun Li <libaokun@xxxxxxxxxxxxxxxxx>

Looks good. Just I think this should be folded in patch 1, since you have
proper Fixes tag, I don't think there's a concern this would be backported
to kernels where rcu_barrier() is still needed.

								Honza

> ---
>  fs/fs-writeback.c | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 1f95ddcee363..9ae290547eb2 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1246,11 +1246,6 @@ void cgroup_writeback_umount(struct super_block *sb)
>  		 * will then drain it.
>  		 */
>  		synchronize_rcu();
> -		/*
> -		 * Use rcu_barrier() to wait for all pending callbacks to
> -		 * ensure that all in-flight wb switches are in the workqueue.
> -		 */
> -		rcu_barrier();
>  		flush_workqueue(isw_wq);
>  	}
>  }
> -- 
> 2.43.7
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR