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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

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