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    SubjectRe: [PATCH 3/3] fs: rcu protect inode hash lookups
    On Mon, Nov 01, 2010 at 10:38:07AM +0100, Eric Dumazet wrote:
    > Le lundi 01 novembre 2010 à 16:33 +1100, Dave Chinner a écrit :
    > > From: Dave Chinner <>
    > >
    > > Now that inodes are using RCU freeing, we can walk the hash lists
    > > using RCU protection during lookups. Convert all the hash list
    > > operations to use RCU-based operators and drop the inode_hash_lock
    > > around pure lookup operations.
    > >
    > > Signed-off-by: Dave Chinner <>
    > You probably should copy Paul on this stuff, I added him in Cc, because
    > SLAB_DESTROY_BY_RCU is really tricky, and Paul review is a must.
    > > repeat:
    > > + rcu_read_lock();
    > > hlist_for_each_entry(inode, node, head, i_hash) {
    > > if (inode->i_sb != sb)
    > > continue;
    > > if (!test(inode, data))
    > > continue;
    > > spin_lock(&inode->i_lock);
    > Problem with SLAB_DESTROY_BY_RCU is the inode can be freed, and reused
    > immediately (no grace period) by another cpu.
    > So you need to recheck test(inode, data) _after_ getting a stable
    > reference on the inode (spin_lock() in this case), to make sure you
    > indeed found the inode you are looking for, not another one.

    Possibly. The test callback is a private callback to determine if,
    indeed, it is the inode the caller is looking for. I need to do a
    deeper look into what ordering is required for this callback.

    > The test on inode->i_sb != sb can be omitted, _if_ each sb has its own
    > kmem_cache (but I am not sure, please check if this is the case)

    There's a slab cache per filesystem type, not per filesystem, so the
    check is necessary.

    > Also, you should make sure the allocation of inode is careful of not
    > overwriting some fields (the i_lock in particular), since you could
    > break a concurrent lookup. This is really tricky, you cannot use
    > spin_lock_init(&inode->i_lock) anymore in inode_init_always().

    Yes, I missed that one. Good catch. I'm used to the XFS code where
    most locks are initialised only once in the slab constructor....

    The other fields of note:

    i_sb: overwritten in inode_init_always(). Should be safe
    simply by rechecking after validating the inode is not in
    the freed state as you suggest.
    i_ino: overwritten just before the inode is re-inserted into
    the hash. redo check like i_sb.
    i_state: initialised atomically with hash insert via i_lock.
    i_hash: inserted into hash list under i_lock

    My intent is that the i_state/i_hash atomicity acts as the real
    guard against reusing a freed inode, but you are right that the
    other fields needs to be rechecked for validity after establishing
    that it is not a freed inode.

    > You can read Documentation/RCU/rculist_nulls.txt for some doc I wrote
    > when adding SLAB_DESTROY_BY_RCU to UDP/TCP sockets. Sockets stable

    Perhaps you should rename that file "slab_destroy_by_rcu-tips.txt",
    because the current name seems unrelated to the contents. :/

    > reference is not a spinlock, but a refcount, so it was easier to init
    > this refcount. With a spinlock, I believe you might need to use SLAB
    > constructor, to initialize the spinlock only on fresh objects, not on
    > reused ones.

    Yeah, that is what I intended.

    Thanks for the comments, Eric.


    Dave Chinner
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