kernel: optimise seqlock

[mirror_ubuntu-bionic-kernel.git] / Documentation / filesystems / dentry-locking.txt

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_lock no longer exists, dentry locking is explained in fs/dcache.c

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. RCU dentry hash walking works like this:

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. This is a standard hlist_rcu iteration.

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(),
   which has since been replaced by hlist_for_each_entry_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 lock protection
   while traversing the hash chain.

3. The dentry looked up without holding locks 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 locks, 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 per-dentry lock (see
   fs/dcache.c). This excludes dput() to ensure that a dentry that has
   been looked up concurrently does not get deleted before dget() can
   take a ref.

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