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

[Jan Kara]

On Mon 18-05-26 21:10:20, Baokun Li wrote:
> 在 2026/5/18 21:01, Jan Kara 写道:
> > 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
> 
> Thank you for your review!
> 
> Because the two patches have different fix tags, there is concern that
> removing rcu_barrier here could cause the stable branches still using
> queue_rcu_work (5.10.y, 6.6.y, ...) to reintroduce similar issues when
> the round of fix patches deletes this necessary synchronization.
> 
> Therefore, the patches are split into two to make their purposes clearer.

Ah, ok, I didn't notice that the first patch has so old fixes tag. OK. Feel
free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								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