Documentation/filesystems/dentry-locking.txt

   1 RCU-based dcache locking model
   2 ==============================
   3
   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.
  12
  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.
  22
  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*.
  33
  34
  35 Dcache locking details
  36 ======================
  37
  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.
  46
  47
  48 Safe lock-free look-up of dcache hash table
  49 ===========================================
  50
  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 :
  56
  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.
  61
  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.
  71
  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.
  81
  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.
  87
  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.
  92
  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.
 105
 106
 107 Maintaining POSIX rename semantics
 108 ==================================
 109
 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 :
 118
 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).
 125
 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.
 132
 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().
 141
 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.
 147
 148
 149 Important guidelines for filesystem developers related to dcache_rcu
 150 ====================================================================
 151
 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.
 157
 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.
 160
 161 3. For a hashed dentry, checking of d_count needs to be protected by
 162    d_lock.
 163
 164
 165 Papers and other documentation on dcache locking
 166 ================================================
 167
 168 1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).
 169
 170 2. http://lse.sourceforge.net/locking/dcache/dcache.html
 171
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 173