--- /dev/null
+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
+
+
+
and the dentry is returned. The caller must use d_put()
to free the dentry when it finishes using it.
-
-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
+For further information on dentry locking, please refer to the document
+Documentation/filesystems/dentry-locking.txt.
Resources
projects / linux-2.6 / commitdiff