[PATCH] VFS: split dentry locking documentation

This patch splits dentry locking documentation from
Documentation/filesystems/vfs.txt to a separate file.  The dentry locking
bits are useful but do not fit into the VFS overview document as is.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 173 insertions, 0 deletions

diff --git a/Documentation/filesystems/dentry-locking.txt b/Documentation/filesystems/dentry-locking.txt
new file mode 100644
index 000000000000..4c0c575a4012
--- /dev/null
+++ b/Documentation/filesystems/dentry-locking.txt
@@ -0,0 +1,173 @@
+RCU-based dcache locking model
+==============================
+
+On many workloads, the most common operation on dcache is to look up a
+dentry, given a parent dentry and the name of the child. Typically,
+for every open(), stat() etc., the dentry corresponding to the
+pathname will be looked up by walking the tree starting with the first
+component of the pathname and using that dentry along with the next
+component to look up the next level and so on. Since it is a frequent
+operation for workloads like multiuser environments and web servers,
+it is important to optimize this path.
+
+Prior to 2.5.10, dcache_lock was acquired in d_lookup and thus in
+every component during path look-up. Since 2.5.10 onwards, fast-walk
+algorithm changed this by holding the dcache_lock at the beginning and
+walking as many cached path component dentries as possible. This
+significantly decreases the number of acquisition of
+dcache_lock. However it also increases the lock hold time
+significantly and affects performance in large SMP machines. Since
+2.5.62 kernel, dcache has been using a new locking model that uses RCU
+to make dcache look-up lock-free.
+
+The current dcache locking model is not very different from the
+existing dcache locking model. Prior to 2.5.62 kernel, dcache_lock
+protected the hash chain, d_child, d_alias, d_lru lists as well as
+d_inode and several other things like mount look-up. RCU-based changes
+affect only the way the hash chain is protected. For everything else
+the dcache_lock must be taken for both traversing as well as
+updating. The hash chain updates too take the dcache_lock.  The
+significant change is the way d_lookup traverses the hash chain, it
+doesn't acquire the dcache_lock for this and rely on RCU to ensure
+that the dentry has not been *freed*.
+
+
+Dcache locking details
+======================
+
+For many multi-user workloads, open() and stat() on files are very
+frequently occurring operations. Both involve walking of path names to
+find the dentry corresponding to the concerned file. In 2.4 kernel,
+dcache_lock was held during look-up of each path component. Contention
+and cache-line bouncing of this global lock caused significant
+scalability problems. With the introduction of RCU in Linux kernel,
+this was worked around by making the look-up of path components during
+path walking lock-free.
+
+
+Safe lock-free look-up of dcache hash table
+===========================================
+
+Dcache is a complex data structure with the hash table entries also
+linked together in other lists. In 2.4 kernel, dcache_lock protected
+all the lists. We applied RCU only on hash chain walking. The rest of
+the lists are still protected by dcache_lock.  Some of the important
+changes are :
+
+1. The deletion from hash chain is done using hlist_del_rcu() macro
+   which doesn't initialize next pointer of the deleted dentry and
+   this allows us to walk safely lock-free while a deletion is
+   happening.
+
+2. Insertion of a dentry into the hash table is done using
+   hlist_add_head_rcu() which take care of ordering the writes - the
+   writes to the dentry must be visible before the dentry is
+   inserted. This works in conjunction with hlist_for_each_rcu() while
+   walking the hash chain. The only requirement is that all
+   initialization to the dentry must be done before
+   hlist_add_head_rcu() since we don't have dcache_lock protection
+   while traversing the hash chain. This isn't different from the
+   existing code.
+
+3. The dentry looked up without holding dcache_lock by cannot be
+   returned for walking if it is unhashed. It then may have a NULL
+   d_inode or other bogosity since RCU doesn't protect the other
+   fields in the dentry. We therefore use a flag DCACHE_UNHASHED to
+   indicate unhashed dentries and use this in conjunction with a
+   per-dentry lock (d_lock). Once looked up without the dcache_lock,
+   we acquire the per-dentry lock (d_lock) and check if the dentry is
+   unhashed. If so, the look-up is failed. If not, the reference count
+   of the dentry is increased and the dentry is returned.
+
+4. Once a dentry is looked up, it must be ensured during the path walk
+   for that component it doesn't go away. In pre-2.5.10 code, this was
+   done holding a reference to the dentry. dcache_rcu does the same.
+   In some sense, dcache_rcu path walking looks like the pre-2.5.10
+   version.
+
+5. All dentry hash chain updates must take the dcache_lock as well as
+   the per-dentry lock in that order. dput() does this to ensure that
+   a dentry that has just been looked up in another CPU doesn't get
+   deleted before dget() can be done on it.
+
+6. There are several ways to do reference counting of RCU protected
+   objects. One such example is in ipv4 route cache where deferred
+   freeing (using call_rcu()) is done as soon as the reference count
+   goes to zero. This cannot be done in the case of dentries because
+   tearing down of dentries require blocking (dentry_iput()) which
+   isn't supported from RCU callbacks. Instead, tearing down of
+   dentries happen synchronously in dput(), but actual freeing happens
+   later when RCU grace period is over. This allows safe lock-free
+   walking of the hash chains, but a matched dentry may have been
+   partially torn down. The checking of DCACHE_UNHASHED flag with
+   d_lock held detects such dentries and prevents them from being
+   returned from look-up.
+
+
+Maintaining POSIX rename semantics
+==================================
+
+Since look-up of dentries is lock-free, it can race against a
+concurrent rename operation. For example, during rename of file A to
+B, look-up of either A or B must succeed.  So, if look-up of B happens
+after A has been removed from the hash chain but not added to the new
+hash chain, it may fail.  Also, a comparison while the name is being
+written concurrently by a rename may result in false positive matches
+violating rename semantics.  Issues related to race with rename are
+handled as described below :
+
+1. Look-up can be done in two ways - d_lookup() which is safe from
+   simultaneous renames and __d_lookup() which is not.  If
+   __d_lookup() fails, it must be followed up by a d_lookup() to
+   correctly determine whether a dentry is in the hash table or
+   not. d_lookup() protects look-ups using a sequence lock
+   (rename_lock).
+
+2. The name associated with a dentry (d_name) may be changed if a
+   rename is allowed to happen simultaneously. To avoid memcmp() in
+   __d_lookup() go out of bounds due to a rename and false positive
+   comparison, the name comparison is done while holding the
+   per-dentry lock. This prevents concurrent renames during this
+   operation.
+
+3. Hash table walking during look-up may move to a different bucket as
+   the current dentry is moved to a different bucket due to rename.
+   But we use hlists in dcache hash table and they are
+   null-terminated.  So, even if a dentry moves to a different bucket,
+   hash chain walk will terminate. [with a list_head list, it may not
+   since termination is when the list_head in the original bucket is
+   reached].  Since we redo the d_parent check and compare name while
+   holding d_lock, lock-free look-up will not race against d_move().
+
+4. There can be a theoretical race when a dentry keeps coming back to
+   original bucket due to double moves. Due to this look-up may
+   consider that it has never moved and can end up in a infinite loop.
+   But this is not any worse that theoretical livelocks we already
+   have in the kernel.
+
+
+Important guidelines for filesystem developers related to dcache_rcu
+====================================================================
+
+1. Existing dcache interfaces (pre-2.5.62) exported to filesystem
+   don't change. Only dcache internal implementation changes. However
+   filesystems *must not* delete from the dentry hash chains directly
+   using the list macros like allowed earlier. They must use dcache
+   APIs like d_drop() or __d_drop() depending on the situation.
+
+2. d_flags is now protected by a per-dentry lock (d_lock). All access
+   to d_flags must be protected by it.
+
+3. For a hashed dentry, checking of d_count needs to be protected by
+   d_lock.
+
+
+Papers and other documentation on dcache locking
+================================================
+
+1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).
+
+2. http://lse.sourceforge.net/locking/dcache/dcache.html
+
+
+