1Path walking and name lookup locking
2====================================
3
4Path resolution is the finding a dentry corresponding to a path name string, by
5performing a path walk. Typically, for every open(), stat() etc., the path name
6will be resolved. Paths are resolved by walking the namespace tree, starting
7with the first component of the pathname (eg. root or cwd) with a known dentry,
8then finding the child of that dentry, which is named the next component in the
9path string. Then repeating the lookup from the child dentry and finding its
10child with the next element, and so on.
11
12Since it is a frequent operation for workloads like multiuser environments and
13web servers, it is important to optimize this code.
14
15Path walking synchronisation history:
16Prior to 2.5.10, dcache_lock was acquired in d_lookup (dcache hash lookup) and
17thus in every component during path look-up. Since 2.5.10 onwards, fast-walk
18algorithm changed this by holding the dcache_lock at the beginning and walking
19as many cached path component dentries as possible. This significantly
20decreases the number of acquisition of dcache_lock. However it also increases
21the lock hold time significantly and affects performance in large SMP machines.
22Since 2.5.62 kernel, dcache has been using a new locking model that uses RCU to
23make dcache look-up lock-free.
24
25All the above algorithms required taking a lock and reference count on the
26dentry that was looked up, so that may be used as the basis for walking the
27next path element. This is inefficient and unscalable. It is inefficient
28because of the locks and atomic operations required for every dentry element
29slows things down. It is not scalable because many parallel applications that
30are path-walk intensive tend to do path lookups starting from a common dentry
31(usually, the root "/" or current working directory). So contention on these
32common path elements causes lock and cacheline queueing.
33
34Since 2.6.38, RCU is used to make a significant part of the entire path walk
35(including dcache look-up) completely "store-free" (so, no locks, atomics, or
36even stores into cachelines of common dentries). This is known as "rcu-walk"
37path walking.
38
39Path walking overview
40=====================
41
42A name string specifies a start (root directory, cwd, fd-relative) and a
43sequence of elements (directory entry names), which together refer to a path in
44the namespace. A path is represented as a (dentry, vfsmount) tuple. The name
45elements are sub-strings, separated by '/'.
46
47Name lookups will want to find a particular path that a name string refers to
48(usually the final element, or parent of final element). This is done by taking
49the path given by the name's starting point (which we know in advance -- eg.
50current->fs->cwd or current->fs->root) as the first parent of the lookup. Then
51iteratively for each subsequent name element, look up the child of the current
52parent with the given name and if it is not the desired entry, make it the
53parent for the next lookup.
54
55A parent, of course, must be a directory, and we must have appropriate
56permissions on the parent inode to be able to walk into it.
57
58Turning the child into a parent for the next lookup requires more checks and
59procedures. Symlinks essentially substitute the symlink name for the target
60name in the name string, and require some recursive path walking.  Mount points
61must be followed into (thus changing the vfsmount that subsequent path elements
62refer to), switching from the mount point path to the root of the particular
63mounted vfsmount. These behaviours are variously modified depending on the
64exact path walking flags.
65
66Path walking then must, broadly, do several particular things:
67- find the start point of the walk;
68- perform permissions and validity checks on inodes;
69- perform dcache hash name lookups on (parent, name element) tuples;
70- traverse mount points;
71- traverse symlinks;
72- lookup and create missing parts of the path on demand.
73
74Safe store-free look-up of dcache hash table
75============================================
76
77Dcache name lookup
78------------------
79In order to lookup a dcache (parent, name) tuple, we take a hash on the tuple
80and use that to select a bucket in the dcache-hash table. The list of entries
81in that bucket is then walked, and we do a full comparison of each entry
82against our (parent, name) tuple.
83
84The hash lists are RCU protected, so list walking is not serialised with
85concurrent updates (insertion, deletion from the hash). This is a standard RCU
86list application with the exception of renames, which will be covered below.
87
88Parent and name members of a dentry, as well as its membership in the dcache
89hash, and its inode are protected by the per-dentry d_lock spinlock. A
90reference is taken on the dentry (while the fields are verified under d_lock),
91and this stabilises its d_inode pointer and actual inode. This gives a stable
92point to perform the next step of our path walk against.
93
94These members are also protected by d_seq seqlock, although this offers
95read-only protection and no durability of results, so care must be taken when
96using d_seq for synchronisation (see seqcount based lookups, below).
97
98Renames
99-------
100Back to the rename case. In usual RCU protected lists, the only operations that
101will happen to an object is insertion, and then eventually removal from the
102list. The object will not be reused until an RCU grace period is complete.
103This ensures the RCU list traversal primitives can run over the object without
104problems (see RCU documentation for how this works).
105
106However when a dentry is renamed, its hash value can change, requiring it to be
107moved to a new hash list. Allocating and inserting a new alias would be
108expensive and also problematic for directory dentries. Latency would be far to
109high to wait for a grace period after removing the dentry and before inserting
110it in the new hash bucket. So what is done is to insert the dentry into the
111new list immediately.
112
113However, when the dentry's list pointers are updated to point to objects in the
114new list before waiting for a grace period, this can result in a concurrent RCU
115lookup of the old list veering off into the new (incorrect) list and missing
116the remaining dentries on the list.
117
118There is no fundamental problem with walking down the wrong list, because the
119dentry comparisons will never match. However it is fatal to miss a matching
120dentry. So a seqlock is used to detect when a rename has occurred, and so the
121lookup can be retried.
122
123         1      2      3
124        +---+  +---+  +---+
125hlist-->| N-+->| N-+->| N-+->
126head <--+-P |<-+-P |<-+-P |
127        +---+  +---+  +---+
128
129Rename of dentry 2 may require it deleted from the above list, and inserted
130into a new list. Deleting 2 gives the following list.
131
132         1             3
133        +---+         +---+     (don't worry, the longer pointers do not
134hlist-->| N-+-------->| N-+->    impose a measurable performance overhead
135head <--+-P |<--------+-P |      on modern CPUs)
136        +---+         +---+
137          ^      2      ^
138          |    +---+    |
139          |    | N-+----+
140          +----+-P |
141               +---+
142
143This is a standard RCU-list deletion, which leaves the deleted object's
144pointers intact, so a concurrent list walker that is currently looking at
145object 2 will correctly continue to object 3 when it is time to traverse the
146next object.
147
148However, when inserting object 2 onto a new list, we end up with this:
149
150         1             3
151        +---+         +---+
152hlist-->| N-+-------->| N-+->
153head <--+-P |<--------+-P |
154        +---+         +---+
155                 2
156               +---+
157               | N-+---->
158          <----+-P |
159               +---+
160
161Because we didn't wait for a grace period, there may be a concurrent lookup
162still at 2. Now when it follows 2's 'next' pointer, it will walk off into
163another list without ever having checked object 3.
164
165A related, but distinctly different, issue is that of rename atomicity versus
166lookup operations. If a file is renamed from 'A' to 'B', a lookup must only
167find either 'A' or 'B'. So if a lookup of 'A' returns NULL, a subsequent lookup
168of 'B' must succeed (note the reverse is not true).
169
170Between deleting the dentry from the old hash list, and inserting it on the new
171hash list, a lookup may find neither 'A' nor 'B' matching the dentry. The same
172rename seqlock is also used to cover this race in much the same way, by
173retrying a negative lookup result if a rename was in progress.
174
175Seqcount based lookups
176----------------------
177In refcount based dcache lookups, d_lock is used to serialise access to
178the dentry, stabilising it while comparing its name and parent and then
179taking a reference count (the reference count then gives a stable place to
180start the next part of the path walk from).
181
182As explained above, we would like to do path walking without taking locks or
183reference counts on intermediate dentries along the path. To do this, a per
184dentry seqlock (d_seq) is used to take a "coherent snapshot" of what the dentry
185looks like (its name, parent, and inode). That snapshot is then used to start
186the next part of the path walk. When loading the coherent snapshot under d_seq,
187care must be taken to load the members up-front, and use those pointers rather
188than reloading from the dentry later on (otherwise we'd have interesting things
189like d_inode going NULL underneath us, if the name was unlinked).
190
191Also important is to avoid performing any destructive operations (pretty much:
192no non-atomic stores to shared data), and to recheck the seqcount when we are
193"done" with the operation. Retry or abort if the seqcount does not match.
194Avoiding destructive or changing operations means we can easily unwind from
195failure.
196
197What this means is that a caller, provided they are holding RCU lock to
198protect the dentry object from disappearing, can perform a seqcount based
199lookup which does not increment the refcount on the dentry or write to
200it in any way. This returned dentry can be used for subsequent operations,
201provided that d_seq is rechecked after that operation is complete.
202
203Inodes are also rcu freed, so the seqcount lookup dentry's inode may also be
204queried for permissions.
205
206With this two parts of the puzzle, we can do path lookups without taking
207locks or refcounts on dentry elements.
208
209RCU-walk path walking design
210============================
211
212Path walking code now has two distinct modes, ref-walk and rcu-walk. ref-walk
213is the traditional[*] way of performing dcache lookups using d_lock to
214serialise concurrent modifications to the dentry and take a reference count on
215it. ref-walk is simple and obvious, and may sleep, take locks, etc while path
216walking is operating on each dentry. rcu-walk uses seqcount based dentry
217lookups, and can perform lookup of intermediate elements without any stores to
218shared data in the dentry or inode. rcu-walk can not be applied to all cases,
219eg. if the filesystem must sleep or perform non trivial operations, rcu-walk
220must be switched to ref-walk mode.
221
222[*] RCU is still used for the dentry hash lookup in ref-walk, but not the full
223    path walk.
224
225Where ref-walk uses a stable, refcounted ``parent'' to walk the remaining
226path string, rcu-walk uses a d_seq protected snapshot. When looking up a
227child of this parent snapshot, we open d_seq critical section on the child
228before closing d_seq critical section on the parent. This gives an interlocking
229ladder of snapshots to walk down.
230
231
232     proc 101
233      /----------------\
234     / comm:    "vi"    \
235    /  fs.root: dentry0  \
236    \  fs.cwd:  dentry2  /
237     \                  /
238      \----------------/
239
240So when vi wants to open("/home/npiggin/test.c", O_RDWR), then it will
241start from current->fs->root, which is a pinned dentry. Alternatively,
242"./test.c" would start from cwd; both names refer to the same path in
243the context of proc101.
244
245     dentry 0
246    +---------------------+   rcu-walk begins here, we note d_seq, check the
247    | name:    "/"        |   inode's permission, and then look up the next
248    | inode:   10         |   path element which is "home"...
249    | children:"home", ...|
250    +---------------------+
251              |
252     dentry 1 V
253    +---------------------+   ... which brings us here. We find dentry1 via
254    | name:    "home"     |   hash lookup, then note d_seq and compare name
255    | inode:   678        |   string and parent pointer. When we have a match,
256    | children:"npiggin"  |   we now recheck the d_seq of dentry0. Then we
257    +---------------------+   check inode and look up the next element.
258              |
259     dentry2  V
260    +---------------------+   Note: if dentry0 is now modified, lookup is
261    | name:    "npiggin"  |   not necessarily invalid, so we need only keep a
262    | inode:   543        |   parent for d_seq verification, and grandparents
263    | children:"a.c", ... |   can be forgotten.
264    +---------------------+
265              |
266     dentry3  V
267    +---------------------+   At this point we have our destination dentry.
268    | name:    "a.c"      |   We now take its d_lock, verify d_seq of this
269    | inode:   14221      |   dentry. If that checks out, we can increment
270    | children:NULL       |   its refcount because we're holding d_lock.
271    +---------------------+
272
273Taking a refcount on a dentry from rcu-walk mode, by taking its d_lock,
274re-checking its d_seq, and then incrementing its refcount is called
275"dropping rcu" or dropping from rcu-walk into ref-walk mode.
276
277It is, in some sense, a bit of a house of cards. If the seqcount check of the
278parent snapshot fails, the house comes down, because we had closed the d_seq
279section on the grandparent, so we have nothing left to stand on. In that case,
280the path walk must be fully restarted (which we do in ref-walk mode, to avoid
281live locks). It is costly to have a full restart, but fortunately they are
282quite rare.
283
284When we reach a point where sleeping is required, or a filesystem callout
285requires ref-walk, then instead of restarting the walk, we attempt to drop rcu
286at the last known good dentry we have. Avoiding a full restart in ref-walk in
287these cases is fundamental for performance and scalability because blocking
288operations such as creates and unlinks are not uncommon.
289
290The detailed design for rcu-walk is like this:
291* LOOKUP_RCU is set in nd->flags, which distinguishes rcu-walk from ref-walk.
292* Take the RCU lock for the entire path walk, starting with the acquiring
293  of the starting path (eg. root/cwd/fd-path). So now dentry refcounts are
294  not required for dentry persistence.
295* synchronize_rcu is called when unregistering a filesystem, so we can
296  access d_ops and i_ops during rcu-walk.
297* Similarly take the vfsmount lock for the entire path walk. So now mnt
298  refcounts are not required for persistence. Also we are free to perform mount
299  lookups, and to assume dentry mount points and mount roots are stable up and
300  down the path.
301* Have a per-dentry seqlock to protect the dentry name, parent, and inode,
302  so we can load this tuple atomically, and also check whether any of its
303  members have changed.
304* Dentry lookups (based on parent, candidate string tuple) recheck the parent
305  sequence after the child is found in case anything changed in the parent
306  during the path walk.
307* inode is also RCU protected so we can load d_inode and use the inode for
308  limited things.
309* i_mode, i_uid, i_gid can be tested for exec permissions during path walk.
310* i_op can be loaded.
311* When the destination dentry is reached, drop rcu there (ie. take d_lock,
312  verify d_seq, increment refcount).
313* If seqlock verification fails anywhere along the path, do a full restart
314  of the path lookup in ref-walk mode. -ECHILD tends to be used (for want of
315  a better errno) to signal an rcu-walk failure.
316
317The cases where rcu-walk cannot continue are:
318* NULL dentry (ie. any uncached path element)
319* Following links
320
321It may be possible eventually to make following links rcu-walk aware.
322
323Uncached path elements will always require dropping to ref-walk mode, at the
324very least because i_mutex needs to be grabbed, and objects allocated.
325
326Final note:
327"store-free" path walking is not strictly store free. We take vfsmount lock
328and refcounts (both of which can be made per-cpu), and we also store to the
329stack (which is essentially CPU-local), and we also have to take locks and
330refcount on final dentry.
331
332The point is that shared data, where practically possible, is not locked
333or stored into. The result is massive improvements in performance and
334scalability of path resolution.
335
336
337Interesting statistics
338======================
339
340The following table gives rcu lookup statistics for a few simple workloads
341(2s12c24t Westmere, debian non-graphical system). Ungraceful are attempts to
342drop rcu that fail due to d_seq failure and requiring the entire path lookup
343again. Other cases are successful rcu-drops that are required before the final
344element, nodentry for missing dentry, revalidate for filesystem revalidate
345routine requiring rcu drop, permission for permission check requiring drop,
346and link for symlink traversal requiring drop.
347
348     rcu-lookups     restart  nodentry          link  revalidate  permission
349bootup     47121           0      4624          1010       10283        7852
350dbench  25386793           0   6778659(26.7%)     55         549        1156
351kbuild   2696672          10     64442(2.3%)  108764(4.0%)     1        1590
352git diff   39605           0        28             2           0         106
353vfstest 24185492        4945    708725(2.9%) 1076136(4.4%)     0        2651
354
355What this shows is that failed rcu-walk lookups, ie. ones that are restarted
356entirely with ref-walk, are quite rare. Even the "vfstest" case which
357specifically has concurrent renames/mkdir/rmdir/ creat/unlink/etc to exercise
358such races is not showing a huge amount of restarts.
359
360Dropping from rcu-walk to ref-walk mean that we have encountered a dentry where
361the reference count needs to be taken for some reason. This is either because
362we have reached the target of the path walk, or because we have encountered a
363condition that can't be resolved in rcu-walk mode.  Ideally, we drop rcu-walk
364only when we have reached the target dentry, so the other statistics show where
365this does not happen.
366
367Note that a graceful drop from rcu-walk mode due to something such as the
368dentry not existing (which can be common) is not necessarily a failure of
369rcu-walk scheme, because some elements of the path may have been walked in
370rcu-walk mode. The further we get from common path elements (such as cwd or
371root), the less contended the dentry is likely to be. The closer we are to
372common path elements, the more likely they will exist in dentry cache.
373
374
375Papers and other documentation on dcache locking
376================================================
377
3781. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).
379
3802. http://lse.sourceforge.net/locking/dcache/dcache.html
381
382
383