Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
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 171 172