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

Al Viro
2011-06-04T00:42:34Z

> @@ -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...