Re: [PATCH 08/12] superblock: introduce per-sb cache shrinker infrastructure

[Al Viro <viro@xxxxxxxxxxxxxxxxxx>]

> @@ -278,7 +325,12 @@ void generic_shutdown_super(struct super_block *sb)
>  {
>       const struct super_operations *sop = sb->s_op;
>  
> -
> +     /*
> +      * shut down the shrinker first so we know that there are no possible
> +      * races when shrinking the dcache or icache. Removes the need for
> +      * external locking to prevent such races.
> +      */
> +     unregister_shrinker(&sb->s_shrink);
>       if (sb->s_root) {
>               shrink_dcache_for_umount(sb);
>               sync_filesystem(sb);

What it means is that shrinker_rwsem now nests inside ->s_umount...  IOW,
if any ->shrink() gets stuck, so does every generic_shutdown_super().
I'm still not convinced it's a good idea - especially since _this_
superblock will be skipped anyway.  Is there any good reason to evict
shrinker that early?  Note that doing that after ->s_umount is dropped
should be reasonably safe - your shrinker will see that superblock is
doomed if it's called anywhere in that window...