From: Jan Kara Subject: Re: [PATCH v2] fs/mbcache: make sure mb_cache_count() not return negative value. Date: Wed, 10 Jan 2018 16:02:23 +0100 Message-ID: <20180110150223.axkqucrhzef2n64u@quack2.suse.cz> References: <1515454691-69220-1-git-send-email-jiang.biao2@zte.com.cn> <20180108161304.f4be912fb6e20cdf56ae78ef@linux-foundation.org> <20180110042600.GC5809@thunk.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andrew Morton , Jiang Biao , linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, ebiggers@google.com, jack@suse.cz, zhong.weidong@zte.com.cn To: Theodore Ts'o Return-path: Received: from mx2.suse.de ([195.135.220.15]:46553 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751932AbeAJPC0 (ORCPT ); Wed, 10 Jan 2018 10:02:26 -0500 Content-Disposition: inline In-Reply-To: <20180110042600.GC5809@thunk.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: 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 > 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 > > 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 SUSE Labs, CR