fs: rcu-walk for path lookup

Perform common cases of path lookups without any stores or locking in the
ancestor dentry elements. This is called rcu-walk, as opposed to the current
algorithm which is a refcount based walk, or ref-walk.

This results in far fewer atomic operations on every path element,
significantly improving path lookup performance. It also avoids cacheline
bouncing on common dentries, significantly improving scalability.

The overall design is like this:
* LOOKUP_RCU is set in nd->flags, which distinguishes rcu-walk from ref-walk.
* Take the RCU lock for the entire path walk, starting with the acquiring
  of the starting path (eg. root/cwd/fd-path). So now dentry refcounts are
  not required for dentry persistence.
* synchronize_rcu is called when unregistering a filesystem, so we can
  access d_ops and i_ops during rcu-walk.
* Similarly take the vfsmount lock for the entire path walk. So now mnt
  refcounts are not required for persistence. Also we are free to perform mount
  lookups, and to assume dentry mount points and mount roots are stable up and
  down the path.
* Have a per-dentry seqlock to protect the dentry name, parent, and inode,
  so we can load this tuple atomically, and also check whether any of its
  members have changed.
* Dentry lookups (based on parent, candidate string tuple) recheck the parent
  sequence after the child is found in case anything changed in the parent
  during the path walk.
* inode is also RCU protected so we can load d_inode and use the inode for
  limited things.
* i_mode, i_uid, i_gid can be tested for exec permissions during path walk.
* i_op can be loaded.

When we reach the destination dentry, we lock it, recheck lookup sequence,
and increment its refcount and mountpoint refcount. RCU and vfsmount locks
are dropped. This is termed "dropping rcu-walk". If the dentry refcount does
not match, we can not drop rcu-walk gracefully at the current point in the
lokup, so instead return -ECHILD (for want of a better errno). This signals the
path walking code to re-do the entire lookup with a ref-walk.

Aside from the final dentry, there are other situations that may be encounted
where we cannot continue rcu-walk. In that case, we drop rcu-walk (ie. take
a reference on the last good dentry) and continue with a ref-walk. Again, if
we can drop rcu-walk gracefully, we return -ECHILD and do the whole lookup
using ref-walk. But it is very important that we can continue with ref-walk
for most cases, particularly to avoid the overhead of double lookups, and to
gain the scalability advantages on common path elements (like cwd and root).

The cases where rcu-walk cannot continue are:
* NULL dentry (ie. any uncached path element)
* parent with d_inode->i_op->permission or ACLs
* dentries with d_revalidate
* Following links

In future patches, permission checks and d_revalidate become rcu-walk aware. It
may be possible eventually to make following links rcu-walk aware.

Uncached path elements will always require dropping to ref-walk mode, at the
very least because i_mutex needs to be grabbed, and objects allocated.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

2 files changed, 345 insertions, 172 deletions

diff --git a/Documentation/filesystems/dentry-locking.txt b/Documentation/filesystems/dentry-locking.txt
deleted file mode 100644
index 30b6a40..0000000
--- a/Documentation/filesystems/dentry-locking.txt
+++ /dev/null
@@ -1,172 +0,0 @@
-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_lock no longer exists, dentry locking is explained in fs/dcache.c
-
-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. RCU dentry hash walking works like this:
-
-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. This is a standard hlist_rcu iteration.
-
-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(),
-   which has since been replaced by hlist_for_each_entry_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 lock protection
-   while traversing the hash chain.
-
-3. The dentry looked up without holding locks 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 locks, 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 per-dentry lock (see
-   fs/dcache.c). This excludes dput() to ensure that a dentry that has
-   been looked up concurrently does not get deleted before dget() can
-   take a ref.
-
-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
-
-
-
diff --git a/Documentation/filesystems/path-lookup.txt b/Documentation/filesystems/path-lookup.txt
new file mode 100644
index 0000000..09b2878
--- /dev/null
+++ b/Documentation/filesystems/path-lookup.txt
@@ -0,0 +1,345 @@
+Path walking and name lookup locking
+====================================
+
+Path resolution is the finding a dentry corresponding to a path name string, by
+performing a path walk. Typically, for every open(), stat() etc., the path name
+will be resolved. Paths are resolved by walking the namespace tree, starting
+with the first component of the pathname (eg. root or cwd) with a known dentry,
+then finding the child of that dentry, which is named the next component in the
+path string. Then repeating the lookup from the child dentry and finding its
+child with the next element, and so on.
+
+Since it is a frequent operation for workloads like multiuser environments and
+web servers, it is important to optimize this code.
+
+Path walking synchronisation history:
+Prior to 2.5.10, dcache_lock was acquired in d_lookup (dcache hash 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.
+
+All the above algorithms required taking a lock and reference count on the
+dentry that was looked up, so that may be used as the basis for walking the
+next path element. This is inefficient and unscalable. It is inefficient
+because of the locks and atomic operations required for every dentry element
+slows things down. It is not scalable because many parallel applications that
+are path-walk intensive tend to do path lookups starting from a common dentry
+(usually, the root "/" or current working directory). So contention on these
+common path elements causes lock and cacheline queueing.
+
+Since 2.6.38, RCU is used to make a significant part of the entire path walk
+(including dcache look-up) completely "store-free" (so, no locks, atomics, or
+even stores into cachelines of common dentries). This is known as "rcu-walk"
+path walking.
+
+Path walking overview
+=====================
+
+A name string specifies a start (root directory, cwd, fd-relative) and a
+sequence of elements (directory entry names), which together refer to a path in
+the namespace. A path is represented as a (dentry, vfsmount) tuple. The name
+elements are sub-strings, seperated by '/'.
+
+Name lookups will want to find a particular path that a name string refers to
+(usually the final element, or parent of final element). This is done by taking
+the path given by the name's starting point (which we know in advance -- eg.
+current->fs->cwd or current->fs->root) as the first parent of the lookup. Then
+iteratively for each subsequent name element, look up the child of the current
+parent with the given name and if it is not the desired entry, make it the
+parent for the next lookup.
+
+A parent, of course, must be a directory, and we must have appropriate
+permissions on the parent inode to be able to walk into it.
+
+Turning the child into a parent for the next lookup requires more checks and
+procedures. Symlinks essentially substitute the symlink name for the target
+name in the name string, and require some recursive path walking.  Mount points
+must be followed into (thus changing the vfsmount that subsequent path elements
+refer to), switching from the mount point path to the root of the particular
+mounted vfsmount. These behaviours are variously modified depending on the
+exact path walking flags.
+
+Path walking then must, broadly, do several particular things:
+- find the start point of the walk;
+- perform permissions and validity checks on inodes;
+- perform dcache hash name lookups on (parent, name element) tuples;
+- traverse mount points;
+- traverse symlinks;
+- lookup and create missing parts of the path on demand.
+
+Safe store-free look-up of dcache hash table
+============================================
+
+Dcache name lookup
+------------------
+In order to lookup a dcache (parent, name) tuple, we take a hash on the tuple
+and use that to select a bucket in the dcache-hash table. The list of entries
+in that bucket is then walked, and we do a full comparison of each entry
+against our (parent, name) tuple.
+
+The hash lists are RCU protected, so list walking is not serialised with
+concurrent updates (insertion, deletion from the hash). This is a standard RCU
+list application with the exception of renames, which will be covered below.
+
+Parent and name members of a dentry, as well as its membership in the dcache
+hash, and its inode are protected by the per-dentry d_lock spinlock. A
+reference is taken on the dentry (while the fields are verified under d_lock),
+and this stabilises its d_inode pointer and actual inode. This gives a stable
+point to perform the next step of our path walk against.
+
+These members are also protected by d_seq seqlock, although this offers
+read-only protection and no durability of results, so care must be taken when
+using d_seq for synchronisation (see seqcount based lookups, below).
+
+Renames
+-------
+Back to the rename case. In usual RCU protected lists, the only operations that
+will happen to an object is insertion, and then eventually removal from the
+list. The object will not be reused until an RCU grace period is complete.
+This ensures the RCU list traversal primitives can run over the object without
+problems (see RCU documentation for how this works).
+
+However when a dentry is renamed, its hash value can change, requiring it to be
+moved to a new hash list. Allocating and inserting a new alias would be
+expensive and also problematic for directory dentries. Latency would be far to
+high to wait for a grace period after removing the dentry and before inserting
+it in the new hash bucket. So what is done is to insert the dentry into the
+new list immediately.
+
+However, when the dentry's list pointers are updated to point to objects in the
+new list before waiting for a grace period, this can result in a concurrent RCU
+lookup of the old list veering off into the new (incorrect) list and missing
+the remaining dentries on the list.
+
+There is no fundamental problem with walking down the wrong list, because the
+dentry comparisons will never match. However it is fatal to miss a matching
+dentry. So a seqlock is used to detect when a rename has occurred, and so the
+lookup can be retried.
+
+         1      2      3
+        +---+  +---+  +---+
+hlist-->| N-+->| N-+->| N-+->
+head <--+-P |<-+-P |<-+-P |
+        +---+  +---+  +---+
+
+Rename of dentry 2 may require it deleted from the above list, and inserted
+into a new list. Deleting 2 gives the following list.
+
+         1             3
+        +---+         +---+     (don't worry, the longer pointers do not
+hlist-->| N-+-------->| N-+->    impose a measurable performance overhead
+head <--+-P |<--------+-P |      on modern CPUs)
+        +---+         +---+
+          ^      2      ^
+          |    +---+    |
+          |    | N-+----+
+          +----+-P |
+               +---+
+
+This is a standard RCU-list deletion, which leaves the deleted object's
+pointers intact, so a concurrent list walker that is currently looking at
+object 2 will correctly continue to object 3 when it is time to traverse the
+next object.
+
+However, when inserting object 2 onto a new list, we end up with this:
+
+         1             3
+        +---+         +---+
+hlist-->| N-+-------->| N-+->
+head <--+-P |<--------+-P |
+        +---+         +---+
+                 2
+               +---+
+               | N-+---->
+          <----+-P |
+               +---+
+
+Because we didn't wait for a grace period, there may be a concurrent lookup
+still at 2. Now when it follows 2's 'next' pointer, it will walk off into
+another list without ever having checked object 3.
+
+A related, but distinctly different, issue is that of rename atomicity versus
+lookup operations. If a file is renamed from 'A' to 'B', a lookup must only
+find either 'A' or 'B'. So if a lookup of 'A' returns NULL, a subsequent lookup
+of 'B' must succeed (note the reverse is not true).
+
+Between deleting the dentry from the old hash list, and inserting it on the new
+hash list, a lookup may find neither 'A' nor 'B' matching the dentry. The same
+rename seqlock is also used to cover this race in much the same way, by
+retrying a negative lookup result if a rename was in progress.
+
+Seqcount based lookups
+----------------------
+In refcount based dcache lookups, d_lock is used to serialise access to
+the dentry, stabilising it while comparing its name and parent and then
+taking a reference count (the reference count then gives a stable place to
+start the next part of the path walk from).
+
+As explained above, we would like to do path walking without taking locks or
+reference counts on intermediate dentries along the path. To do this, a per
+dentry seqlock (d_seq) is used to take a "coherent snapshot" of what the dentry
+looks like (its name, parent, and inode). That snapshot is then used to start
+the next part of the path walk. When loading the coherent snapshot under d_seq,
+care must be taken to load the members up-front, and use those pointers rather
+than reloading from the dentry later on (otherwise we'd have interesting things
+like d_inode going NULL underneath us, if the name was unlinked).
+
+Also important is to avoid performing any destructive operations (pretty much:
+no non-atomic stores to shared data), and to recheck the seqcount when we are
+"done" with the operation. Retry or abort if the seqcount does not match.
+Avoiding destructive or changing operations means we can easily unwind from
+failure.
+
+What this means is that a caller, provided they are holding RCU lock to
+protect the dentry object from disappearing, can perform a seqcount based
+lookup which does not increment the refcount on the dentry or write to
+it in any way. This returned dentry can be used for subsequent operations,
+provided that d_seq is rechecked after that operation is complete.
+
+Inodes are also rcu freed, so the seqcount lookup dentry's inode may also be
+queried for permissions.
+
+With this two parts of the puzzle, we can do path lookups without taking
+locks or refcounts on dentry elements.
+
+RCU-walk path walking design
+============================
+
+Path walking code now has two distinct modes, ref-walk and rcu-walk. ref-walk
+is the traditional[*] way of performing dcache lookups using d_lock to
+serialise concurrent modifications to the dentry and take a reference count on
+it. ref-walk is simple and obvious, and may sleep, take locks, etc while path
+walking is operating on each dentry. rcu-walk uses seqcount based dentry
+lookups, and can perform lookup of intermediate elements without any stores to
+shared data in the dentry or inode. rcu-walk can not be applied to all cases,
+eg. if the filesystem must sleep or perform non trivial operations, rcu-walk
+must be switched to ref-walk mode.
+
+[*] RCU is still used for the dentry hash lookup in ref-walk, but not the full
+    path walk.
+
+Where ref-walk uses a stable, refcounted ``parent'' to walk the remaining
+path string, rcu-walk uses a d_seq protected snapshot. When looking up a
+child of this parent snapshot, we open d_seq critical section on the child
+before closing d_seq critical section on the parent. This gives an interlocking
+ladder of snapshots to walk down.
+
+
+     proc 101
+      /----------------\
+     / comm:    "vi"    \
+    /  fs.root: dentry0  \
+    \  fs.cwd:  dentry2  /
+     \                  /
+      \----------------/
+
+So when vi wants to open("/home/npiggin/test.c", O_RDWR), then it will
+start from current->fs->root, which is a pinned dentry. Alternatively,
+"./test.c" would start from cwd; both names refer to the same path in
+the context of proc101.
+
+     dentry 0
+    +---------------------+   rcu-walk begins here, we note d_seq, check the
+    | name:    "/"        |   inode's permission, and then look up the next
+    | inode:   10         |   path element which is "home"...
+    | children:"home", ...|
+    +---------------------+
+              |
+     dentry 1 V
+    +---------------------+   ... which brings us here. We find dentry1 via
+    | name:    "home"     |   hash lookup, then note d_seq and compare name
+    | inode:   678        |   string and parent pointer. When we have a match,
+    | children:"npiggin"  |   we now recheck the d_seq of dentry0. Then we
+    +---------------------+   check inode and look up the next element.
+              |
+     dentry2  V
+    +---------------------+   Note: if dentry0 is now modified, lookup is
+    | name:    "npiggin"  |   not necessarily invalid, so we need only keep a
+    | inode:   543        |   parent for d_seq verification, and grandparents
+    | children:"a.c", ... |   can be forgotten.
+    +---------------------+
+              |
+     dentry3  V
+    +---------------------+   At this point we have our destination dentry.
+    | name:    "a.c"      |   We now take its d_lock, verify d_seq of this
+    | inode:   14221      |   dentry. If that checks out, we can increment
+    | children:NULL       |   its refcount because we're holding d_lock.
+    +---------------------+
+
+Taking a refcount on a dentry from rcu-walk mode, by taking its d_lock,
+re-checking its d_seq, and then incrementing its refcount is called
+"dropping rcu" or dropping from rcu-walk into ref-walk mode.
+
+It is, in some sense, a bit of a house of cards. If the seqcount check of the
+parent snapshot fails, the house comes down, because we had closed the d_seq
+section on the grandparent, so we have nothing left to stand on. In that case,
+the path walk must be fully restarted (which we do in ref-walk mode, to avoid
+live locks). It is costly to have a full restart, but fortunately they are
+quite rare.
+
+When we reach a point where sleeping is required, or a filesystem callout
+requires ref-walk, then instead of restarting the walk, we attempt to drop rcu
+at the last known good dentry we have. Avoiding a full restart in ref-walk in
+these cases is fundamental for performance and scalability because blocking
+operations such as creates and unlinks are not uncommon.
+
+The detailed design for rcu-walk is like this:
+* LOOKUP_RCU is set in nd->flags, which distinguishes rcu-walk from ref-walk.
+* Take the RCU lock for the entire path walk, starting with the acquiring
+  of the starting path (eg. root/cwd/fd-path). So now dentry refcounts are
+  not required for dentry persistence.
+* synchronize_rcu is called when unregistering a filesystem, so we can
+  access d_ops and i_ops during rcu-walk.
+* Similarly take the vfsmount lock for the entire path walk. So now mnt
+  refcounts are not required for persistence. Also we are free to perform mount
+  lookups, and to assume dentry mount points and mount roots are stable up and
+  down the path.
+* Have a per-dentry seqlock to protect the dentry name, parent, and inode,
+  so we can load this tuple atomically, and also check whether any of its
+  members have changed.
+* Dentry lookups (based on parent, candidate string tuple) recheck the parent
+  sequence after the child is found in case anything changed in the parent
+  during the path walk.
+* inode is also RCU protected so we can load d_inode and use the inode for
+  limited things.
+* i_mode, i_uid, i_gid can be tested for exec permissions during path walk.
+* i_op can be loaded.
+* When the destination dentry is reached, drop rcu there (ie. take d_lock,
+  verify d_seq, increment refcount).
+* If seqlock verification fails anywhere along the path, do a full restart
+  of the path lookup in ref-walk mode. -ECHILD tends to be used (for want of
+  a better errno) to signal an rcu-walk failure.
+
+The cases where rcu-walk cannot continue are:
+* NULL dentry (ie. any uncached path element)
+* parent with d_inode->i_op->permission or ACLs
+* dentries with d_revalidate
+* Following links
+
+In future patches, permission checks and d_revalidate become rcu-walk aware. It
+may be possible eventually to make following links rcu-walk aware.
+
+Uncached path elements will always require dropping to ref-walk mode, at the
+very least because i_mutex needs to be grabbed, and objects allocated.
+
+Final note:
+"store-free" path walking is not strictly store free. We take vfsmount lock
+and refcounts (both of which can be made per-cpu), and we also store to the
+stack (which is essentially CPU-local), and we also have to take locks and
+refcount on final dentry.
+
+The point is that shared data, where practically possible, is not locked
+or stored into. The result is massive improvements in performance and
+scalability of path resolution.
+
+
+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