175 lines
8.2 KiB
Plaintext
175 lines
8.2 KiB
Plaintext
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(),
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which has since been replaced by hlist_for_each_entry_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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