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+Path walking and name lookup locking
+Path resolution is the finding a dentry corresponding to a path name string, by
+performing a path walk. Typically, for every open(), stat() etc., the path name
+will be resolved. Paths are resolved by walking the namespace tree, starting
+with the first component of the pathname (eg. root or cwd) with a known dentry,
+then finding the child of that dentry, which is named the next component in the
+path string. Then repeating the lookup from the child dentry and finding its
+child with the next element, and so on.
+Since it is a frequent operation for workloads like multiuser environments and
+web servers, it is important to optimize this code.
+Path walking synchronisation history:
+Prior to 2.5.10, dcache_lock was acquired in d_lookup (dcache hash lookup) and
+thus in every component during path look-up. Since 2.5.10 onwards, fast-walk
+algorithm changed this by holding the dcache_lock at the beginning and walking
+as many cached path component dentries as possible. This significantly
+decreases the number of acquisition of dcache_lock. However it also increases
+the lock hold time significantly and affects performance in large SMP machines.
+Since 2.5.62 kernel, dcache has been using a new locking model that uses RCU to
+make dcache look-up lock-free.
+All the above algorithms required taking a lock and reference count on the
+dentry that was looked up, so that may be used as the basis for walking the
+next path element. This is inefficient and unscalable. It is inefficient
+because of the locks and atomic operations required for every dentry element
+slows things down. It is not scalable because many parallel applications that
+are path-walk intensive tend to do path lookups starting from a common dentry
+(usually, the root "/" or current working directory). So contention on these
+common path elements causes lock and cacheline queueing.
+Since 2.6.38, RCU is used to make a significant part of the entire path walk
+(including dcache look-up) completely "store-free" (so, no locks, atomics, or
+even stores into cachelines of common dentries). This is known as "rcu-walk"
+path walking.
+Path walking overview
+A name string specifies a start (root directory, cwd, fd-relative) and a
+sequence of elements (directory entry names), which together refer to a path in
+the namespace. A path is represented as a (dentry, vfsmount) tuple. The name
+elements are sub-strings, separated by '/'.
+Name lookups will want to find a particular path that a name string refers to
+(usually the final element, or parent of final element). This is done by taking
+the path given by the name's starting point (which we know in advance -- eg.
+current->fs->cwd or current->fs->root) as the first parent of the lookup. Then
+iteratively for each subsequent name element, look up the child of the current
+parent with the given name and if it is not the desired entry, make it the
+parent for the next lookup.
+A parent, of course, must be a directory, and we must have appropriate
+permissions on the parent inode to be able to walk into it.
+Turning the child into a parent for the next lookup requires more checks and
+procedures. Symlinks essentially substitute the symlink name for the target
+name in the name string, and require some recursive path walking. Mount points
+must be followed into (thus changing the vfsmount that subsequent path elements
+refer to), switching from the mount point path to the root of the particular
+mounted vfsmount. These behaviours are variously modified depending on the
+exact path walking flags.
+Path walking then must, broadly, do several particular things:
+- find the start point of the walk;
+- perform permissions and validity checks on inodes;
+- perform dcache hash name lookups on (parent, name element) tuples;
+- traverse mount points;
+- traverse symlinks;
+- lookup and create missing parts of the path on demand.
+Safe store-free look-up of dcache hash table
+Dcache name lookup
+In order to lookup a dcache (parent, name) tuple, we take a hash on the tuple
+and use that to select a bucket in the dcache-hash table. The list of entries
+in that bucket is then walked, and we do a full comparison of each entry
+against our (parent, name) tuple.
+The hash lists are RCU protected, so list walking is not serialised with
+concurrent updates (insertion, deletion from the hash). This is a standard RCU
+list application with the exception of renames, which will be covered below.
+Parent and name members of a dentry, as well as its membership in the dcache
+hash, and its inode are protected by the per-dentry d_lock spinlock. A
+reference is taken on the dentry (while the fields are verified under d_lock),
+and this stabilises its d_inode pointer and actual inode. This gives a stable
+point to perform the next step of our path walk against.
+These members are also protected by d_seq seqlock, although this offers
+read-only protection and no durability of results, so care must be taken when
+using d_seq for synchronisation (see seqcount based lookups, below).
+Back to the rename case. In usual RCU protected lists, the only operations that
+will happen to an object is insertion, and then eventually removal from the
+list. The object will not be reused until an RCU grace period is complete.
+This ensures the RCU list traversal primitives can run over the object without
+problems (see RCU documentation for how this works).
+However when a dentry is renamed, its hash value can change, requiring it to be
+moved to a new hash list. Allocating and inserting a new alias would be
+expensive and also problematic for directory dentries. Latency would be far to
+high to wait for a grace period after removing the dentry and before inserting
+it in the new hash bucket. So what is done is to insert the dentry into the
+new list immediately.
+However, when the dentry's list pointers are updated to point to objects in the
+new list before waiting for a grace period, this can result in a concurrent RCU
+lookup of the old list veering off into the new (incorrect) list and missing
+the remaining dentries on the list.
+There is no fundamental problem with walking down the wrong list, because the
+dentry comparisons will never match. However it is fatal to miss a matching
+dentry. So a seqlock is used to detect when a rename has occurred, and so the
+lookup can be retried.
+ 1 2 3
+ +---+ +---+ +---+
+hlist-->| N-+->| N-+->| N-+->
+head <--+-P |<-+-P |<-+-P |
+ +---+ +---+ +---+
+Rename of dentry 2 may require it deleted from the above list, and inserted
+into a new list. Deleting 2 gives the following list.
+ 1 3
+ +---+ +---+ (don't worry, the longer pointers do not
+hlist-->| N-+-------->| N-+-> impose a measurable performance overhead
+head <--+-P |<--------+-P | on modern CPUs)
+ +---+ +---+
+ ^ 2 ^
+ | +---+ |
+ | | N-+----+
+ +----+-P |
+ +---+
+This is a standard RCU-list deletion, which leaves the deleted object's
+pointers intact, so a concurrent list walker that is currently looking at
+object 2 will correctly continue to object 3 when it is time to traverse the
+next object.
+However, when inserting object 2 onto a new list, we end up with this:
+ 1 3
+ +---+ +---+
+hlist-->| N-+-------->| N-+->
+head <--+-P |<--------+-P |
+ +---+ +---+
+ 2
+ +---+
+ | N-+---->
+ <----+-P |
+ +---+
+Because we didn't wait for a grace period, there may be a concurrent lookup
+still at 2. Now when it follows 2's 'next' pointer, it will walk off into
+another list without ever having checked object 3.
+A related, but distinctly different, issue is that of rename atomicity versus
+lookup operations. If a file is renamed from 'A' to 'B', a lookup must only
+find either 'A' or 'B'. So if a lookup of 'A' returns NULL, a subsequent lookup
+of 'B' must succeed (note the reverse is not true).
+Between deleting the dentry from the old hash list, and inserting it on the new
+hash list, a lookup may find neither 'A' nor 'B' matching the dentry. The same
+rename seqlock is also used to cover this race in much the same way, by
+retrying a negative lookup result if a rename was in progress.
+Seqcount based lookups
+In refcount based dcache lookups, d_lock is used to serialise access to
+the dentry, stabilising it while comparing its name and parent and then
+taking a reference count (the reference count then gives a stable place to
+start the next part of the path walk from).
+As explained above, we would like to do path walking without taking locks or
+reference counts on intermediate dentries along the path. To do this, a per
+dentry seqlock (d_seq) is used to take a "coherent snapshot" of what the dentry
+looks like (its name, parent, and inode). That snapshot is then used to start
+the next part of the path walk. When loading the coherent snapshot under d_seq,
+care must be taken to load the members up-front, and use those pointers rather
+than reloading from the dentry later on (otherwise we'd have interesting things
+like d_inode going NULL underneath us, if the name was unlinked).
+Also important is to avoid performing any destructive operations (pretty much:
+no non-atomic stores to shared data), and to recheck the seqcount when we are
+"done" with the operation. Retry or abort if the seqcount does not match.
+Avoiding destructive or changing operations means we can easily unwind from
+What this means is that a caller, provided they are holding RCU lock to
+protect the dentry object from disappearing, can perform a seqcount based
+lookup which does not increment the refcount on the dentry or write to
+it in any way. This returned dentry can be used for subsequent operations,
+provided that d_seq is rechecked after that operation is complete.
+Inodes are also rcu freed, so the seqcount lookup dentry's inode may also be
+queried for permissions.
+With this two parts of the puzzle, we can do path lookups without taking
+locks or refcounts on dentry elements.
+RCU-walk path walking design
+Path walking code now has two distinct modes, ref-walk and rcu-walk. ref-walk
+is the traditional[*] way of performing dcache lookups using d_lock to
+serialise concurrent modifications to the dentry and take a reference count on
+it. ref-walk is simple and obvious, and may sleep, take locks, etc while path
+walking is operating on each dentry. rcu-walk uses seqcount based dentry
+lookups, and can perform lookup of intermediate elements without any stores to
+shared data in the dentry or inode. rcu-walk can not be applied to all cases,
+eg. if the filesystem must sleep or perform non trivial operations, rcu-walk
+must be switched to ref-walk mode.
+[*] RCU is still used for the dentry hash lookup in ref-walk, but not the full
+ path walk.
+Where ref-walk uses a stable, refcounted ``parent'' to walk the remaining
+path string, rcu-walk uses a d_seq protected snapshot. When looking up a
+child of this parent snapshot, we open d_seq critical section on the child
+before closing d_seq critical section on the parent. This gives an interlocking
+ladder of snapshots to walk down.
+ proc 101
+ /----------------\
+ / comm: "vi" \
+ / fs.root: dentry0 \
+ \ fs.cwd: dentry2 /
+ \ /
+ \----------------/
+So when vi wants to open("/home/npiggin/test.c", O_RDWR), then it will
+start from current->fs->root, which is a pinned dentry. Alternatively,
+"./test.c" would start from cwd; both names refer to the same path in
+the context of proc101.
+ dentry 0
+ +---------------------+ rcu-walk begins here, we note d_seq, check the
+ | name: "/" | inode's permission, and then look up the next
+ | inode: 10 | path element which is "home"...
+ | children:"home", ...|
+ +---------------------+
+ |
+ dentry 1 V
+ +---------------------+ ... which brings us here. We find dentry1 via
+ | name: "home" | hash lookup, then note d_seq and compare name
+ | inode: 678 | string and parent pointer. When we have a match,
+ | children:"npiggin" | we now recheck the d_seq of dentry0. Then we
+ +---------------------+ check inode and look up the next element.
+ |
+ dentry2 V
+ +---------------------+ Note: if dentry0 is now modified, lookup is
+ | name: "npiggin" | not necessarily invalid, so we need only keep a
+ | inode: 543 | parent for d_seq verification, and grandparents
+ | children:"a.c", ... | can be forgotten.
+ +---------------------+
+ |
+ dentry3 V
+ +---------------------+ At this point we have our destination dentry.
+ | name: "a.c" | We now take its d_lock, verify d_seq of this
+ | inode: 14221 | dentry. If that checks out, we can increment
+ | children:NULL | its refcount because we're holding d_lock.
+ +---------------------+
+Taking a refcount on a dentry from rcu-walk mode, by taking its d_lock,
+re-checking its d_seq, and then incrementing its refcount is called
+"dropping rcu" or dropping from rcu-walk into ref-walk mode.
+It is, in some sense, a bit of a house of cards. If the seqcount check of the
+parent snapshot fails, the house comes down, because we had closed the d_seq
+section on the grandparent, so we have nothing left to stand on. In that case,
+the path walk must be fully restarted (which we do in ref-walk mode, to avoid
+live locks). It is costly to have a full restart, but fortunately they are
+quite rare.
+When we reach a point where sleeping is required, or a filesystem callout
+requires ref-walk, then instead of restarting the walk, we attempt to drop rcu
+at the last known good dentry we have. Avoiding a full restart in ref-walk in
+these cases is fundamental for performance and scalability because blocking
+operations such as creates and unlinks are not uncommon.
+The detailed design for rcu-walk is like this:
+* LOOKUP_RCU is set in nd->flags, which distinguishes rcu-walk from ref-walk.
+* Take the RCU lock for the entire path walk, starting with the acquiring
+ of the starting path (eg. root/cwd/fd-path). So now dentry refcounts are
+ not required for dentry persistence.
+* synchronize_rcu is called when unregistering a filesystem, so we can
+ access d_ops and i_ops during rcu-walk.
+* Similarly take the vfsmount lock for the entire path walk. So now mnt
+ refcounts are not required for persistence. Also we are free to perform mount
+ lookups, and to assume dentry mount points and mount roots are stable up and
+ down the path.
+* Have a per-dentry seqlock to protect the dentry name, parent, and inode,
+ so we can load this tuple atomically, and also check whether any of its
+ members have changed.
+* Dentry lookups (based on parent, candidate string tuple) recheck the parent
+ sequence after the child is found in case anything changed in the parent
+ during the path walk.
+* inode is also RCU protected so we can load d_inode and use the inode for
+ limited things.
+* i_mode, i_uid, i_gid can be tested for exec permissions during path walk.
+* i_op can be loaded.
+* When the destination dentry is reached, drop rcu there (ie. take d_lock,
+ verify d_seq, increment refcount).
+* If seqlock verification fails anywhere along the path, do a full restart
+ of the path lookup in ref-walk mode. -ECHILD tends to be used (for want of
+ a better errno) to signal an rcu-walk failure.
+The cases where rcu-walk cannot continue are:
+* NULL dentry (ie. any uncached path element)
+* Following links
+It may be possible eventually to make following links rcu-walk aware.
+Uncached path elements will always require dropping to ref-walk mode, at the
+very least because i_mutex needs to be grabbed, and objects allocated.
+Final note:
+"store-free" path walking is not strictly store free. We take vfsmount lock
+and refcounts (both of which can be made per-cpu), and we also store to the
+stack (which is essentially CPU-local), and we also have to take locks and
+refcount on final dentry.
+The point is that shared data, where practically possible, is not locked
+or stored into. The result is massive improvements in performance and
+scalability of path resolution.
+Interesting statistics
+The following table gives rcu lookup statistics for a few simple workloads
+(2s12c24t Westmere, debian non-graphical system). Ungraceful are attempts to
+drop rcu that fail due to d_seq failure and requiring the entire path lookup
+again. Other cases are successful rcu-drops that are required before the final
+element, nodentry for missing dentry, revalidate for filesystem revalidate
+routine requiring rcu drop, permission for permission check requiring drop,
+and link for symlink traversal requiring drop.
+ rcu-lookups restart nodentry link revalidate permission
+bootup 47121 0 4624 1010 10283 7852
+dbench 25386793 0 6778659(26.7%) 55 549 1156
+kbuild 2696672 10 64442(2.3%) 108764(4.0%) 1 1590
+git diff 39605 0 28 2 0 106
+vfstest 24185492 4945 708725(2.9%) 1076136(4.4%) 0 2651
+What this shows is that failed rcu-walk lookups, ie. ones that are restarted
+entirely with ref-walk, are quite rare. Even the "vfstest" case which
+specifically has concurrent renames/mkdir/rmdir/ creat/unlink/etc to exercise
+such races is not showing a huge amount of restarts.
+Dropping from rcu-walk to ref-walk mean that we have encountered a dentry where
+the reference count needs to be taken for some reason. This is either because
+we have reached the target of the path walk, or because we have encountered a
+condition that can't be resolved in rcu-walk mode. Ideally, we drop rcu-walk
+only when we have reached the target dentry, so the other statistics show where
+this does not happen.
+Note that a graceful drop from rcu-walk mode due to something such as the
+dentry not existing (which can be common) is not necessarily a failure of
+rcu-walk scheme, because some elements of the path may have been walked in
+rcu-walk mode. The further we get from common path elements (such as cwd or
+root), the less contended the dentry is likely to be. The closer we are to
+common path elements, the more likely they will exist in dentry cache.
+Papers and other documentation on dcache locking
+1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).
+2. http://lse.sourceforge.net/locking/dcache/dcache.html