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