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diff --git a/Documentation/filesystems/dentry-locking.txt b/Documentation/filesystems/dentry-locking.txt new file mode 100644 index 0000000..4c0c575 --- /dev/null +++ b/Documentation/filesystems/dentry-locking.txt @@ -0,0 +1,173 @@ +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 + + + |
