1Path walking and name lookup locking 2==================================== 3 4Path resolution is the finding a dentry corresponding to a path name string, by 5performing a path walk. Typically, for every open(), stat() etc., the path name 6will be resolved. Paths are resolved by walking the namespace tree, starting 7with the first component of the pathname (eg. root or cwd) with a known dentry, 8then finding the child of that dentry, which is named the next component in the 9path string. Then repeating the lookup from the child dentry and finding its 10child with the next element, and so on. 11 12Since it is a frequent operation for workloads like multiuser environments and 13web servers, it is important to optimize this code. 14 15Path walking synchronisation history: 16Prior to 2.5.10, dcache_lock was acquired in d_lookup (dcache hash lookup) and 17thus in every component during path look-up. Since 2.5.10 onwards, fast-walk 18algorithm changed this by holding the dcache_lock at the beginning and walking 19as many cached path component dentries as possible. This significantly 20decreases the number of acquisition of dcache_lock. However it also increases 21the lock hold time significantly and affects performance in large SMP machines. 22Since 2.5.62 kernel, dcache has been using a new locking model that uses RCU to 23make dcache look-up lock-free. 24 25All the above algorithms required taking a lock and reference count on the 26dentry that was looked up, so that may be used as the basis for walking the 27next path element. This is inefficient and unscalable. It is inefficient 28because of the locks and atomic operations required for every dentry element 29slows things down. It is not scalable because many parallel applications that 30are 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 32common path elements causes lock and cacheline queueing. 33 34Since 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 36even stores into cachelines of common dentries). This is known as "rcu-walk" 37path walking. 38 39Path walking overview 40===================== 41 42A name string specifies a start (root directory, cwd, fd-relative) and a 43sequence of elements (directory entry names), which together refer to a path in 44the namespace. A path is represented as a (dentry, vfsmount) tuple. The name 45elements are sub-strings, separated by '/'. 46 47Name 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 49the path given by the name's starting point (which we know in advance -- eg. 50current->fs->cwd or current->fs->root) as the first parent of the lookup. Then 51iteratively for each subsequent name element, look up the child of the current 52parent with the given name and if it is not the desired entry, make it the 53parent for the next lookup. 54 55A parent, of course, must be a directory, and we must have appropriate 56permissions on the parent inode to be able to walk into it. 57 58Turning the child into a parent for the next lookup requires more checks and 59procedures. Symlinks essentially substitute the symlink name for the target 60name in the name string, and require some recursive path walking. Mount points 61must be followed into (thus changing the vfsmount that subsequent path elements 62refer to), switching from the mount point path to the root of the particular 63mounted vfsmount. These behaviours are variously modified depending on the 64exact path walking flags. 65 66Path 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. 73 74Safe store-free look-up of dcache hash table 75============================================ 76 77Dcache name lookup 78------------------ 79In order to lookup a dcache (parent, name) tuple, we take a hash on the tuple 80and use that to select a bucket in the dcache-hash table. The list of entries 81in that bucket is then walked, and we do a full comparison of each entry 82against our (parent, name) tuple. 83 84The hash lists are RCU protected, so list walking is not serialised with 85concurrent updates (insertion, deletion from the hash). This is a standard RCU 86list application with the exception of renames, which will be covered below. 87 88Parent and name members of a dentry, as well as its membership in the dcache 89hash, and its inode are protected by the per-dentry d_lock spinlock. A 90reference is taken on the dentry (while the fields are verified under d_lock), 91and this stabilises its d_inode pointer and actual inode. This gives a stable 92point to perform the next step of our path walk against. 93 94These members are also protected by d_seq seqlock, although this offers 95read-only protection and no durability of results, so care must be taken when 96using d_seq for synchronisation (see seqcount based lookups, below). 97 98Renames 99------- 100Back to the rename case. In usual RCU protected lists, the only operations that 101will happen to an object is insertion, and then eventually removal from the 102list. The object will not be reused until an RCU grace period is complete. 103This ensures the RCU list traversal primitives can run over the object without 104problems (see RCU documentation for how this works). 105 106However when a dentry is renamed, its hash value can change, requiring it to be 107moved to a new hash list. Allocating and inserting a new alias would be 108expensive and also problematic for directory dentries. Latency would be far to 109high to wait for a grace period after removing the dentry and before inserting 110it in the new hash bucket. So what is done is to insert the dentry into the 111new list immediately. 112 113However, when the dentry's list pointers are updated to point to objects in the 114new list before waiting for a grace period, this can result in a concurrent RCU 115lookup of the old list veering off into the new (incorrect) list and missing 116the remaining dentries on the list. 117 118There is no fundamental problem with walking down the wrong list, because the 119dentry comparisons will never match. However it is fatal to miss a matching 120dentry. So a seqlock is used to detect when a rename has occurred, and so the 121lookup can be retried. 122 123 1 2 3 124 +---+ +---+ +---+ 125hlist-->| N-+->| N-+->| N-+-> 126head <--+-P |<-+-P |<-+-P | 127 +---+ +---+ +---+ 128 129Rename of dentry 2 may require it deleted from the above list, and inserted 130into a new list. Deleting 2 gives the following list. 131 132 1 3 133 +---+ +---+ (don't worry, the longer pointers do not 134hlist-->| N-+-------->| N-+-> impose a measurable performance overhead 135head <--+-P |<--------+-P | on modern CPUs) 136 +---+ +---+ 137 ^ 2 ^ 138 | +---+ | 139 | | N-+----+ 140 +----+-P | 141 +---+ 142 143This is a standard RCU-list deletion, which leaves the deleted object's 144pointers intact, so a concurrent list walker that is currently looking at 145object 2 will correctly continue to object 3 when it is time to traverse the 146next object. 147 148However, when inserting object 2 onto a new list, we end up with this: 149 150 1 3 151 +---+ +---+ 152hlist-->| N-+-------->| N-+-> 153head <--+-P |<--------+-P | 154 +---+ +---+ 155 2 156 +---+ 157 | N-+----> 158 <----+-P | 159 +---+ 160 161Because we didn't wait for a grace period, there may be a concurrent lookup 162still at 2. Now when it follows 2's 'next' pointer, it will walk off into 163another list without ever having checked object 3. 164 165A related, but distinctly different, issue is that of rename atomicity versus 166lookup operations. If a file is renamed from 'A' to 'B', a lookup must only 167find either 'A' or 'B'. So if a lookup of 'A' returns NULL, a subsequent lookup 168of 'B' must succeed (note the reverse is not true). 169 170Between deleting the dentry from the old hash list, and inserting it on the new 171hash list, a lookup may find neither 'A' nor 'B' matching the dentry. The same 172rename seqlock is also used to cover this race in much the same way, by 173retrying a negative lookup result if a rename was in progress. 174 175Seqcount based lookups 176---------------------- 177In refcount based dcache lookups, d_lock is used to serialise access to 178the dentry, stabilising it while comparing its name and parent and then 179taking a reference count (the reference count then gives a stable place to 180start the next part of the path walk from). 181 182As explained above, we would like to do path walking without taking locks or 183reference counts on intermediate dentries along the path. To do this, a per 184dentry seqlock (d_seq) is used to take a "coherent snapshot" of what the dentry 185looks like (its name, parent, and inode). That snapshot is then used to start 186the next part of the path walk. When loading the coherent snapshot under d_seq, 187care must be taken to load the members up-front, and use those pointers rather 188than reloading from the dentry later on (otherwise we'd have interesting things 189like d_inode going NULL underneath us, if the name was unlinked). 190 191Also important is to avoid performing any destructive operations (pretty much: 192no 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. 194Avoiding destructive or changing operations means we can easily unwind from 195failure. 196 197What this means is that a caller, provided they are holding RCU lock to 198protect the dentry object from disappearing, can perform a seqcount based 199lookup which does not increment the refcount on the dentry or write to 200it in any way. This returned dentry can be used for subsequent operations, 201provided that d_seq is rechecked after that operation is complete. 202 203Inodes are also rcu freed, so the seqcount lookup dentry's inode may also be 204queried for permissions. 205 206With this two parts of the puzzle, we can do path lookups without taking 207locks or refcounts on dentry elements. 208 209RCU-walk path walking design 210============================ 211 212Path walking code now has two distinct modes, ref-walk and rcu-walk. ref-walk 213is the traditional[*] way of performing dcache lookups using d_lock to 214serialise concurrent modifications to the dentry and take a reference count on 215it. ref-walk is simple and obvious, and may sleep, take locks, etc while path 216walking is operating on each dentry. rcu-walk uses seqcount based dentry 217lookups, and can perform lookup of intermediate elements without any stores to 218shared data in the dentry or inode. rcu-walk can not be applied to all cases, 219eg. if the filesystem must sleep or perform non trivial operations, rcu-walk 220must be switched to ref-walk mode. 221 222[*] RCU is still used for the dentry hash lookup in ref-walk, but not the full 223 path walk. 224 225Where ref-walk uses a stable, refcounted ``parent'' to walk the remaining 226path string, rcu-walk uses a d_seq protected snapshot. When looking up a 227child of this parent snapshot, we open d_seq critical section on the child 228before closing d_seq critical section on the parent. This gives an interlocking 229ladder of snapshots to walk down. 230 231 232 proc 101 233 /----------------\ 234 / comm: "vi" \ 235 / fs.root: dentry0 \ 236 \ fs.cwd: dentry2 / 237 \ / 238 \----------------/ 239 240So when vi wants to open("/home/npiggin/test.c", O_RDWR), then it will 241start 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 243the context of proc101. 244 245 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 +---------------------+ 272 273Taking a refcount on a dentry from rcu-walk mode, by taking its d_lock, 274re-checking its d_seq, and then incrementing its refcount is called 275"dropping rcu" or dropping from rcu-walk into ref-walk mode. 276 277It is, in some sense, a bit of a house of cards. If the seqcount check of the 278parent snapshot fails, the house comes down, because we had closed the d_seq 279section on the grandparent, so we have nothing left to stand on. In that case, 280the path walk must be fully restarted (which we do in ref-walk mode, to avoid 281live locks). It is costly to have a full restart, but fortunately they are 282quite rare. 283 284When we reach a point where sleeping is required, or a filesystem callout 285requires ref-walk, then instead of restarting the walk, we attempt to drop rcu 286at the last known good dentry we have. Avoiding a full restart in ref-walk in 287these cases is fundamental for performance and scalability because blocking 288operations such as creates and unlinks are not uncommon. 289 290The 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. 316 317The cases where rcu-walk cannot continue are: 318* NULL dentry (ie. any uncached path element) 319* Following links 320 321It may be possible eventually to make following links rcu-walk aware. 322 323Uncached path elements will always require dropping to ref-walk mode, at the 324very least because i_mutex needs to be grabbed, and objects allocated. 325 326Final note: 327"store-free" path walking is not strictly store free. We take vfsmount lock 328and refcounts (both of which can be made per-cpu), and we also store to the 329stack (which is essentially CPU-local), and we also have to take locks and 330refcount on final dentry. 331 332The point is that shared data, where practically possible, is not locked 333or stored into. The result is massive improvements in performance and 334scalability of path resolution. 335 336 337Interesting statistics 338====================== 339 340The following table gives rcu lookup statistics for a few simple workloads 341(2s12c24t Westmere, debian non-graphical system). Ungraceful are attempts to 342drop rcu that fail due to d_seq failure and requiring the entire path lookup 343again. Other cases are successful rcu-drops that are required before the final 344element, nodentry for missing dentry, revalidate for filesystem revalidate 345routine requiring rcu drop, permission for permission check requiring drop, 346and link for symlink traversal requiring drop. 347 348 rcu-lookups restart nodentry link revalidate permission 349bootup 47121 0 4624 1010 10283 7852 350dbench 25386793 0 6778659(26.7%) 55 549 1156 351kbuild 2696672 10 64442(2.3%) 108764(4.0%) 1 1590 352git diff 39605 0 28 2 0 106 353vfstest 24185492 4945 708725(2.9%) 1076136(4.4%) 0 2651 354 355What this shows is that failed rcu-walk lookups, ie. ones that are restarted 356entirely with ref-walk, are quite rare. Even the "vfstest" case which 357specifically has concurrent renames/mkdir/rmdir/ creat/unlink/etc to exercise 358such races is not showing a huge amount of restarts. 359 360Dropping from rcu-walk to ref-walk mean that we have encountered a dentry where 361the reference count needs to be taken for some reason. This is either because 362we have reached the target of the path walk, or because we have encountered a 363condition that can't be resolved in rcu-walk mode. Ideally, we drop rcu-walk 364only when we have reached the target dentry, so the other statistics show where 365this does not happen. 366 367Note that a graceful drop from rcu-walk mode due to something such as the 368dentry not existing (which can be common) is not necessarily a failure of 369rcu-walk scheme, because some elements of the path may have been walked in 370rcu-walk mode. The further we get from common path elements (such as cwd or 371root), the less contended the dentry is likely to be. The closer we are to 372common path elements, the more likely they will exist in dentry cache. 373 374 375Papers and other documentation on dcache locking 376================================================ 377 3781. Scaling dcache with RCU (https://linuxjournal.com/article.php?sid=7124). 379 3802. http://lse.sourceforge.net/locking/dcache/dcache.html 381 3823. path-lookup.md in this directory. 383