mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-19 18:53:52 +08:00
cbf8f0f36a
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>
174 lines
8.2 KiB
Plaintext
174 lines
8.2 KiB
Plaintext
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
|
|
|
|
|
|
|