382 lines
18 KiB
Text
382 lines
18 KiB
Text
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).
|
|
|
|
Renames
|
|
-------
|
|
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
|
|
failure.
|
|
|
|
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
|
|
|
|
|