view Documentation/filesystems/dentry-locking.txt @ 524:7f8b544237bf

netfront: Allow netfront in domain 0.

This is useful if your physical network device is in a utility domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
line source
1 RCU-based dcache locking model
2 ==============================
4 On many workloads, the most common operation on dcache is to look up a
5 dentry, given a parent dentry and the name of the child. Typically,
6 for every open(), stat() etc., the dentry corresponding to the
7 pathname will be looked up by walking the tree starting with the first
8 component of the pathname and using that dentry along with the next
9 component to look up the next level and so on. Since it is a frequent
10 operation for workloads like multiuser environments and web servers,
11 it is important to optimize this path.
13 Prior to 2.5.10, dcache_lock was acquired in d_lookup and thus in
14 every component during path look-up. Since 2.5.10 onwards, fast-walk
15 algorithm changed this by holding the dcache_lock at the beginning and
16 walking as many cached path component dentries as possible. This
17 significantly decreases the number of acquisition of
18 dcache_lock. However it also increases the lock hold time
19 significantly and affects performance in large SMP machines. Since
20 2.5.62 kernel, dcache has been using a new locking model that uses RCU
21 to make dcache look-up lock-free.
23 The current dcache locking model is not very different from the
24 existing dcache locking model. Prior to 2.5.62 kernel, dcache_lock
25 protected the hash chain, d_child, d_alias, d_lru lists as well as
26 d_inode and several other things like mount look-up. RCU-based changes
27 affect only the way the hash chain is protected. For everything else
28 the dcache_lock must be taken for both traversing as well as
29 updating. The hash chain updates too take the dcache_lock. The
30 significant change is the way d_lookup traverses the hash chain, it
31 doesn't acquire the dcache_lock for this and rely on RCU to ensure
32 that the dentry has not been *freed*.
35 Dcache locking details
36 ======================
38 For many multi-user workloads, open() and stat() on files are very
39 frequently occurring operations. Both involve walking of path names to
40 find the dentry corresponding to the concerned file. In 2.4 kernel,
41 dcache_lock was held during look-up of each path component. Contention
42 and cache-line bouncing of this global lock caused significant
43 scalability problems. With the introduction of RCU in Linux kernel,
44 this was worked around by making the look-up of path components during
45 path walking lock-free.
48 Safe lock-free look-up of dcache hash table
49 ===========================================
51 Dcache is a complex data structure with the hash table entries also
52 linked together in other lists. In 2.4 kernel, dcache_lock protected
53 all the lists. We applied RCU only on hash chain walking. The rest of
54 the lists are still protected by dcache_lock. Some of the important
55 changes are :
57 1. The deletion from hash chain is done using hlist_del_rcu() macro
58 which doesn't initialize next pointer of the deleted dentry and
59 this allows us to walk safely lock-free while a deletion is
60 happening.
62 2. Insertion of a dentry into the hash table is done using
63 hlist_add_head_rcu() which take care of ordering the writes - the
64 writes to the dentry must be visible before the dentry is
65 inserted. This works in conjunction with hlist_for_each_rcu() while
66 walking the hash chain. The only requirement is that all
67 initialization to the dentry must be done before
68 hlist_add_head_rcu() since we don't have dcache_lock protection
69 while traversing the hash chain. This isn't different from the
70 existing code.
72 3. The dentry looked up without holding dcache_lock by cannot be
73 returned for walking if it is unhashed. It then may have a NULL
74 d_inode or other bogosity since RCU doesn't protect the other
75 fields in the dentry. We therefore use a flag DCACHE_UNHASHED to
76 indicate unhashed dentries and use this in conjunction with a
77 per-dentry lock (d_lock). Once looked up without the dcache_lock,
78 we acquire the per-dentry lock (d_lock) and check if the dentry is
79 unhashed. If so, the look-up is failed. If not, the reference count
80 of the dentry is increased and the dentry is returned.
82 4. Once a dentry is looked up, it must be ensured during the path walk
83 for that component it doesn't go away. In pre-2.5.10 code, this was
84 done holding a reference to the dentry. dcache_rcu does the same.
85 In some sense, dcache_rcu path walking looks like the pre-2.5.10
86 version.
88 5. All dentry hash chain updates must take the dcache_lock as well as
89 the per-dentry lock in that order. dput() does this to ensure that
90 a dentry that has just been looked up in another CPU doesn't get
91 deleted before dget() can be done on it.
93 6. There are several ways to do reference counting of RCU protected
94 objects. One such example is in ipv4 route cache where deferred
95 freeing (using call_rcu()) is done as soon as the reference count
96 goes to zero. This cannot be done in the case of dentries because
97 tearing down of dentries require blocking (dentry_iput()) which
98 isn't supported from RCU callbacks. Instead, tearing down of
99 dentries happen synchronously in dput(), but actual freeing happens
100 later when RCU grace period is over. This allows safe lock-free
101 walking of the hash chains, but a matched dentry may have been
102 partially torn down. The checking of DCACHE_UNHASHED flag with
103 d_lock held detects such dentries and prevents them from being
104 returned from look-up.
107 Maintaining POSIX rename semantics
108 ==================================
110 Since look-up of dentries is lock-free, it can race against a
111 concurrent rename operation. For example, during rename of file A to
112 B, look-up of either A or B must succeed. So, if look-up of B happens
113 after A has been removed from the hash chain but not added to the new
114 hash chain, it may fail. Also, a comparison while the name is being
115 written concurrently by a rename may result in false positive matches
116 violating rename semantics. Issues related to race with rename are
117 handled as described below :
119 1. Look-up can be done in two ways - d_lookup() which is safe from
120 simultaneous renames and __d_lookup() which is not. If
121 __d_lookup() fails, it must be followed up by a d_lookup() to
122 correctly determine whether a dentry is in the hash table or
123 not. d_lookup() protects look-ups using a sequence lock
124 (rename_lock).
126 2. The name associated with a dentry (d_name) may be changed if a
127 rename is allowed to happen simultaneously. To avoid memcmp() in
128 __d_lookup() go out of bounds due to a rename and false positive
129 comparison, the name comparison is done while holding the
130 per-dentry lock. This prevents concurrent renames during this
131 operation.
133 3. Hash table walking during look-up may move to a different bucket as
134 the current dentry is moved to a different bucket due to rename.
135 But we use hlists in dcache hash table and they are
136 null-terminated. So, even if a dentry moves to a different bucket,
137 hash chain walk will terminate. [with a list_head list, it may not
138 since termination is when the list_head in the original bucket is
139 reached]. Since we redo the d_parent check and compare name while
140 holding d_lock, lock-free look-up will not race against d_move().
142 4. There can be a theoretical race when a dentry keeps coming back to
143 original bucket due to double moves. Due to this look-up may
144 consider that it has never moved and can end up in a infinite loop.
145 But this is not any worse that theoretical livelocks we already
146 have in the kernel.
149 Important guidelines for filesystem developers related to dcache_rcu
150 ====================================================================
152 1. Existing dcache interfaces (pre-2.5.62) exported to filesystem
153 don't change. Only dcache internal implementation changes. However
154 filesystems *must not* delete from the dentry hash chains directly
155 using the list macros like allowed earlier. They must use dcache
156 APIs like d_drop() or __d_drop() depending on the situation.
158 2. d_flags is now protected by a per-dentry lock (d_lock). All access
159 to d_flags must be protected by it.
161 3. For a hashed dentry, checking of d_count needs to be protected by
162 d_lock.
165 Papers and other documentation on dcache locking
166 ================================================
168 1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).
170 2. http://lse.sourceforge.net/locking/dcache/dcache.html