linux-ext4 - Re: [PATCH v2] fs/mbcache: make sure mb_cache

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Message-ID: <20180110201137.GB6499@thunk.org>
Date:   Wed, 10 Jan 2018 15:11:37 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jiang Biao <jiang.biao2@....com.cn>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        ebiggers@...gle.com, zhong.weidong@....com.cn
Subject: Re: [PATCH v2] fs/mbcache: make sure mb_cache_count() not return
 negative value.

On Wed, Jan 10, 2018 at 04:02:23PM +0100, Jan Kara wrote:
> 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.

Hmm... you're right.  How we handle the hlist_bl_lock and c_list_lock
still creeps me out a bit, but it's not going to cause the potential
problem.  I think there is a problem if mb_cache_entry_create() races
with mb_cache_delete(), but that will result in an entry being on the
c_list while not being on the hash list, and it doesn't cause the
c_entry_count to get out of sync with reality.

Drat....

						- Ted