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[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>
This commit is contained in:
Pekka Enberg
Linus Torvalds
parent
cc7d1f8f96
commit
cbf8f0f36a
2 changed files with 175 additions and 175 deletions
173
Documentation/filesystems/dentry-locking.txt
Normal file
173
Documentation/filesystems/dentry-locking.txt
Normal file
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@ -0,0 +1,173 @@
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RCU-based dcache locking model
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==============================
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On many workloads, the most common operation on dcache is to look up a
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dentry, given a parent dentry and the name of the child. Typically,
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for every open(), stat() etc., the dentry corresponding to the
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pathname will be looked up by walking the tree starting with the first
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component of the pathname and using that dentry along with the next
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component to look up the next level and so on. Since it is a frequent
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operation for workloads like multiuser environments and web servers,
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it is important to optimize this path.
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Prior to 2.5.10, dcache_lock was acquired in d_lookup and thus in
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every component during path look-up. Since 2.5.10 onwards, fast-walk
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algorithm changed this by holding the dcache_lock at the beginning and
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walking as many cached path component dentries as possible. This
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significantly decreases the number of acquisition of
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dcache_lock. However it also increases the lock hold time
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significantly and affects performance in large SMP machines. Since
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2.5.62 kernel, dcache has been using a new locking model that uses RCU
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to make dcache look-up lock-free.
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The current dcache locking model is not very different from the
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existing dcache locking model. Prior to 2.5.62 kernel, dcache_lock
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protected the hash chain, d_child, d_alias, d_lru lists as well as
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d_inode and several other things like mount look-up. RCU-based changes
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affect only the way the hash chain is protected. For everything else
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the dcache_lock must be taken for both traversing as well as
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updating. The hash chain updates too take the dcache_lock. The
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significant change is the way d_lookup traverses the hash chain, it
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doesn't acquire the dcache_lock for this and rely on RCU to ensure
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that the dentry has not been *freed*.
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Dcache locking details
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======================
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For many multi-user workloads, open() and stat() on files are very
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frequently occurring operations. Both involve walking of path names to
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find the dentry corresponding to the concerned file. In 2.4 kernel,
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dcache_lock was held during look-up of each path component. Contention
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and cache-line bouncing of this global lock caused significant
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scalability problems. With the introduction of RCU in Linux kernel,
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this was worked around by making the look-up of path components during
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path walking lock-free.
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Safe lock-free look-up of dcache hash table
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===========================================
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Dcache is a complex data structure with the hash table entries also
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linked together in other lists. In 2.4 kernel, dcache_lock protected
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all the lists. We applied RCU only on hash chain walking. The rest of
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the lists are still protected by dcache_lock. Some of the important
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changes are :
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1. The deletion from hash chain is done using hlist_del_rcu() macro
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which doesn't initialize next pointer of the deleted dentry and
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this allows us to walk safely lock-free while a deletion is
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happening.
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2. Insertion of a dentry into the hash table is done using
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hlist_add_head_rcu() which take care of ordering the writes - the
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writes to the dentry must be visible before the dentry is
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inserted. This works in conjunction with hlist_for_each_rcu() while
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walking the hash chain. The only requirement is that all
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initialization to the dentry must be done before
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hlist_add_head_rcu() since we don't have dcache_lock protection
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while traversing the hash chain. This isn't different from the
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existing code.
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3. The dentry looked up without holding dcache_lock by cannot be
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returned for walking if it is unhashed. It then may have a NULL
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d_inode or other bogosity since RCU doesn't protect the other
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fields in the dentry. We therefore use a flag DCACHE_UNHASHED to
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indicate unhashed dentries and use this in conjunction with a
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per-dentry lock (d_lock). Once looked up without the dcache_lock,
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we acquire the per-dentry lock (d_lock) and check if the dentry is
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unhashed. If so, the look-up is failed. If not, the reference count
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of the dentry is increased and the dentry is returned.
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4. Once a dentry is looked up, it must be ensured during the path walk
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for that component it doesn't go away. In pre-2.5.10 code, this was
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done holding a reference to the dentry. dcache_rcu does the same.
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In some sense, dcache_rcu path walking looks like the pre-2.5.10
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version.
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5. All dentry hash chain updates must take the dcache_lock as well as
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the per-dentry lock in that order. dput() does this to ensure that
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a dentry that has just been looked up in another CPU doesn't get
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deleted before dget() can be done on it.
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6. There are several ways to do reference counting of RCU protected
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objects. One such example is in ipv4 route cache where deferred
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freeing (using call_rcu()) is done as soon as the reference count
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goes to zero. This cannot be done in the case of dentries because
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tearing down of dentries require blocking (dentry_iput()) which
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isn't supported from RCU callbacks. Instead, tearing down of
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dentries happen synchronously in dput(), but actual freeing happens
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later when RCU grace period is over. This allows safe lock-free
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walking of the hash chains, but a matched dentry may have been
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partially torn down. The checking of DCACHE_UNHASHED flag with
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d_lock held detects such dentries and prevents them from being
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returned from look-up.
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Maintaining POSIX rename semantics
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==================================
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Since look-up of dentries is lock-free, it can race against a
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concurrent rename operation. For example, during rename of file A to
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B, look-up of either A or B must succeed. So, if look-up of B happens
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after A has been removed from the hash chain but not added to the new
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hash chain, it may fail. Also, a comparison while the name is being
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written concurrently by a rename may result in false positive matches
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violating rename semantics. Issues related to race with rename are
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handled as described below :
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1. Look-up can be done in two ways - d_lookup() which is safe from
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simultaneous renames and __d_lookup() which is not. If
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__d_lookup() fails, it must be followed up by a d_lookup() to
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correctly determine whether a dentry is in the hash table or
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not. d_lookup() protects look-ups using a sequence lock
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(rename_lock).
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2. The name associated with a dentry (d_name) may be changed if a
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rename is allowed to happen simultaneously. To avoid memcmp() in
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__d_lookup() go out of bounds due to a rename and false positive
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comparison, the name comparison is done while holding the
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per-dentry lock. This prevents concurrent renames during this
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operation.
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3. Hash table walking during look-up may move to a different bucket as
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the current dentry is moved to a different bucket due to rename.
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But we use hlists in dcache hash table and they are
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null-terminated. So, even if a dentry moves to a different bucket,
|
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hash chain walk will terminate. [with a list_head list, it may not
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since termination is when the list_head in the original bucket is
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reached]. Since we redo the d_parent check and compare name while
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holding d_lock, lock-free look-up will not race against d_move().
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4. There can be a theoretical race when a dentry keeps coming back to
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original bucket due to double moves. Due to this look-up may
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consider that it has never moved and can end up in a infinite loop.
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But this is not any worse that theoretical livelocks we already
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have in the kernel.
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Important guidelines for filesystem developers related to dcache_rcu
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====================================================================
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1. Existing dcache interfaces (pre-2.5.62) exported to filesystem
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don't change. Only dcache internal implementation changes. However
|
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filesystems *must not* delete from the dentry hash chains directly
|
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using the list macros like allowed earlier. They must use dcache
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APIs like d_drop() or __d_drop() depending on the situation.
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2. d_flags is now protected by a per-dentry lock (d_lock). All access
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to d_flags must be protected by it.
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3. For a hashed dentry, checking of d_count needs to be protected by
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d_lock.
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Papers and other documentation on dcache locking
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================================================
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1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).
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2. http://lse.sourceforge.net/locking/dcache/dcache.html
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@ -721,181 +721,8 @@ manipulate dentries:
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and the dentry is returned. The caller must use d_put()
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to free the dentry when it finishes using it.
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RCU-based dcache locking model
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------------------------------
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On many workloads, the most common operation on dcache is
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to look up a dentry, given a parent dentry and the name
|
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of the child. Typically, for every open(), stat() etc.,
|
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the dentry corresponding to the pathname will be looked
|
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up by walking the tree starting with the first component
|
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of the pathname and using that dentry along with the next
|
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component to look up the next level and so on. Since it
|
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is a frequent operation for workloads like multiuser
|
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environments and web servers, it is important to optimize
|
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this path.
|
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|
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Prior to 2.5.10, dcache_lock was acquired in d_lookup and thus
|
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in every component during path look-up. Since 2.5.10 onwards,
|
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fast-walk algorithm changed this by holding the dcache_lock
|
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at the beginning and walking as many cached path component
|
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dentries as possible. This significantly decreases the number
|
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of acquisition of dcache_lock. However it also increases the
|
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lock hold time significantly and affects performance in large
|
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SMP machines. Since 2.5.62 kernel, dcache has been using
|
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a new locking model that uses RCU to make dcache look-up
|
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lock-free.
|
||||
|
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The current dcache locking model is not very different from the existing
|
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dcache locking model. Prior to 2.5.62 kernel, dcache_lock
|
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protected the hash chain, d_child, d_alias, d_lru lists as well
|
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as d_inode and several other things like mount look-up. RCU-based
|
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changes affect only the way the hash chain is protected. For everything
|
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else the dcache_lock must be taken for both traversing as well as
|
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updating. The hash chain updates too take the dcache_lock.
|
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The significant change is the way d_lookup traverses the hash chain,
|
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it doesn't acquire the dcache_lock for this and rely on RCU to
|
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ensure that the dentry has not been *freed*.
|
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|
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Dcache locking details
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----------------------
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|
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For many multi-user workloads, open() and stat() on files are
|
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very frequently occurring operations. Both involve walking
|
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of path names to find the dentry corresponding to the
|
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concerned file. In 2.4 kernel, dcache_lock was held
|
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during look-up of each path component. Contention and
|
||||
cache-line bouncing of this global lock caused significant
|
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scalability problems. With the introduction of RCU
|
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in Linux kernel, this was worked around by making
|
||||
the look-up of path components during path walking lock-free.
|
||||
|
||||
|
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Safe lock-free look-up of dcache hash table
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===========================================
|
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|
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Dcache is a complex data structure with the hash table entries
|
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also linked together in other lists. In 2.4 kernel, dcache_lock
|
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protected all the lists. We applied RCU only on hash chain
|
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walking. The rest of the lists are still protected by dcache_lock.
|
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Some of the important changes are :
|
||||
|
||||
1. The deletion from hash chain is done using hlist_del_rcu() macro which
|
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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 -
|
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the writes to the dentry must be visible before the dentry
|
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is inserted. This works in conjunction with hlist_for_each_rcu()
|
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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
|
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the hash chain. This isn't different from the existing code.
|
||||
|
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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
|
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indicate unhashed dentries and use this in conjunction with a
|
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per-dentry lock (d_lock). Once looked up without the dcache_lock,
|
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we acquire the per-dentry lock (d_lock) and check if the
|
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dentry is unhashed. If so, the look-up is failed. If not, the
|
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reference count of the dentry is increased and the dentry is returned.
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|
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4. Once a dentry is looked up, it must be ensured during the path
|
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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.
|
||||
|
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5. All dentry hash chain updates must take the dcache_lock as well as
|
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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.
|
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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.
|
||||
|
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2. d_flags is now protected by a per-dentry lock (d_lock). All
|
||||
access to d_flags must be protected by it.
|
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|
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3. For a hashed dentry, checking of d_count needs to be protected
|
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by d_lock.
|
||||
|
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|
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Papers and other documentation on dcache locking
|
||||
================================================
|
||||
|
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1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).
|
||||
|
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2. http://lse.sourceforge.net/locking/dcache/dcache.html
|
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For further information on dentry locking, please refer to the document
|
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Documentation/filesystems/dentry-locking.txt.
|
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|
||||
|
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Resources
|
||||
|
|
|
|||
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