Based on kernel version 2.6.37. Page generated on 2011-03-22 21:56 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(), 66 which has since been replaced by hlist_for_each_entry_rcu(), while 67 walking the hash chain. The only requirement is that all 68 initialization to the dentry must be done before 69 hlist_add_head_rcu() since we don't have dcache_lock protection 70 while traversing the hash chain. This isn't different from the 71 existing code. 72 73 3. The dentry looked up without holding dcache_lock by cannot be 74 returned for walking if it is unhashed. It then may have a NULL 75 d_inode or other bogosity since RCU doesn't protect the other 76 fields in the dentry. We therefore use a flag DCACHE_UNHASHED to 77 indicate unhashed dentries and use this in conjunction with a 78 per-dentry lock (d_lock). Once looked up without the dcache_lock, 79 we acquire the per-dentry lock (d_lock) and check if the dentry is 80 unhashed. If so, the look-up is failed. If not, the reference count 81 of the dentry is increased and the dentry is returned. 82 83 4. Once a dentry is looked up, it must be ensured during the path walk 84 for that component it doesn't go away. In pre-2.5.10 code, this was 85 done holding a reference to the dentry. dcache_rcu does the same. 86 In some sense, dcache_rcu path walking looks like the pre-2.5.10 87 version. 88 89 5. All dentry hash chain updates must take the dcache_lock as well as 90 the per-dentry lock in that order. dput() does this to ensure that 91 a dentry that has just been looked up in another CPU doesn't get 92 deleted before dget() can be done on it. 93 94 6. There are several ways to do reference counting of RCU protected 95 objects. One such example is in ipv4 route cache where deferred 96 freeing (using call_rcu()) is done as soon as the reference count 97 goes to zero. This cannot be done in the case of dentries because 98 tearing down of dentries require blocking (dentry_iput()) which 99 isn't supported from RCU callbacks. Instead, tearing down of 100 dentries happen synchronously in dput(), but actual freeing happens 101 later when RCU grace period is over. This allows safe lock-free 102 walking of the hash chains, but a matched dentry may have been 103 partially torn down. The checking of DCACHE_UNHASHED flag with 104 d_lock held detects such dentries and prevents them from being 105 returned from look-up. 106 107 108 Maintaining POSIX rename semantics 109 ================================== 110 111 Since look-up of dentries is lock-free, it can race against a 112 concurrent rename operation. For example, during rename of file A to 113 B, look-up of either A or B must succeed. So, if look-up of B happens 114 after A has been removed from the hash chain but not added to the new 115 hash chain, it may fail. Also, a comparison while the name is being 116 written concurrently by a rename may result in false positive matches 117 violating rename semantics. Issues related to race with rename are 118 handled as described below : 119 120 1. Look-up can be done in two ways - d_lookup() which is safe from 121 simultaneous renames and __d_lookup() which is not. If 122 __d_lookup() fails, it must be followed up by a d_lookup() to 123 correctly determine whether a dentry is in the hash table or 124 not. d_lookup() protects look-ups using a sequence lock 125 (rename_lock). 126 127 2. The name associated with a dentry (d_name) may be changed if a 128 rename is allowed to happen simultaneously. To avoid memcmp() in 129 __d_lookup() go out of bounds due to a rename and false positive 130 comparison, the name comparison is done while holding the 131 per-dentry lock. This prevents concurrent renames during this 132 operation. 133 134 3. Hash table walking during look-up may move to a different bucket as 135 the current dentry is moved to a different bucket due to rename. 136 But we use hlists in dcache hash table and they are 137 null-terminated. So, even if a dentry moves to a different bucket, 138 hash chain walk will terminate. [with a list_head list, it may not 139 since termination is when the list_head in the original bucket is 140 reached]. Since we redo the d_parent check and compare name while 141 holding d_lock, lock-free look-up will not race against d_move(). 142 143 4. There can be a theoretical race when a dentry keeps coming back to 144 original bucket due to double moves. Due to this look-up may 145 consider that it has never moved and can end up in a infinite loop. 146 But this is not any worse that theoretical livelocks we already 147 have in the kernel. 148 149 150 Important guidelines for filesystem developers related to dcache_rcu 151 ==================================================================== 152 153 1. Existing dcache interfaces (pre-2.5.62) exported to filesystem 154 don't change. Only dcache internal implementation changes. However 155 filesystems *must not* delete from the dentry hash chains directly 156 using the list macros like allowed earlier. They must use dcache 157 APIs like d_drop() or __d_drop() depending on the situation. 158 159 2. d_flags is now protected by a per-dentry lock (d_lock). All access 160 to d_flags must be protected by it. 161 162 3. For a hashed dentry, checking of d_count needs to be protected by 163 d_lock. 164 165 166 Papers and other documentation on dcache locking 167 ================================================ 168 169 1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124). 170 171 2. http://lse.sourceforge.net/locking/dcache/dcache.html 172 173