On Tue 09-01-18 23:26:01, Theodore Ts'o wrote:
> On Mon, Jan 08, 2018 at 04:13:04PM -0800, Andrew Morton wrote:
> > I agree with Jan's comment. We need to figure out how ->c_entry_count
> > went negative. mb_cache_count() says this state is "Unlikely, but not
> > impossible", but from a quick read I can't see how this happens - it
> > appears that coherency between ->c_list and ->c_entry_count is always
> > maintained under ->c_list_lock?
>
> I think I see the problem; and I think this should fix it. Andrew,
> Jan, can you review and double check my analysis?
>
> Thanks,
>
> - Ted
>
> commit 18fb3649c7cd9e70f05045656c1888459d96dfe4
> Author: Theodore Ts'o <tytso@xxxxxxx>
> Date: Tue Jan 9 23:24:53 2018 -0500
>
> mbcache: fix potential double counting when removing entry
>
> Entries are removed from the mb_cache entry in two places:
> mb_cache_shrink() and mb_cache_entry_delete(). The mb_cache_shrink()
> function finds the entry to delete via the cache->c_list pointer,
> while mb_cache_entry_delete() finds the entry via the hash lists.
>
> If the two functions race with each other, trying to delete an entry
> at the same time, it's possible for cache->c_entry_count to get
> decremented twice for that one entry. Fix this by checking to see if
> entry is still on the cache list before removing it and dropping
> c_entry_count.
So I don't think this can be a problem. Look, mb_cache_shrink() holds
c_list_lock. It will take first entry from cache->c_list - this list is
using list_head entry->e_list and so we are guaranteed entry->e_list is
non-empty.
The other place deleting entry - mb_cache_entry_delete() - which is using
different list to grab the entry is properly checking for
!list_empty(entry->e_list) after acquiring c_list_lock.
Honza
>
> Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
>
> diff --git a/fs/mbcache.c b/fs/mbcache.c
> index 49c5b25bfa8c..0851af5c1c3d 100644
> --- a/fs/mbcache.c
> +++ b/fs/mbcache.c
> @@ -290,8 +290,10 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
> list_move_tail(&entry->e_list, &cache->c_list);
> continue;
> }
> - list_del_init(&entry->e_list);
> - cache->c_entry_count--;
> + if (!list_empty(&entry->e_list)) {
> + list_del_init(&entry->e_list);
> + cache->c_entry_count--;
> + }
> /*
> * We keep LRU list reference so that entry doesn't go away
> * from under us.
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
