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Message-Id: <8da9284bb667d842193760c1e9cd40abbf587a5c.1586960617.git.mchehab+huawei@kernel.org>
Date: Wed, 15 Apr 2020 16:32:36 +0200
From: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
To: Linux Doc Mailing List <linux-doc@...r.kernel.org>
Cc: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
Alexander Viro <viro@...iv.linux.org.uk>,
linux-fsdevel@...r.kernel.org
Subject: [PATCH 23/34] docs: filesystems: convert path-walking.txt to ReST
- Add a SPDX header;
- Add a document title;
- Adjust document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to filesystems/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
---
Documentation/filesystems/index.rst | 1 +
.../{path-walking.txt => path-walking.rst} | 88 ++++++++++---------
Documentation/filesystems/porting.rst | 2 +-
fs/dcache.c | 6 +-
fs/namei.c | 2 +-
5 files changed, 54 insertions(+), 45 deletions(-)
rename Documentation/filesystems/{path-walking.txt => path-walking.rst} (91%)
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 4d63eafc9fbf..a35b7d6ee5bc 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -20,6 +20,7 @@ algorithms work.
vfs
path-lookup
+ path-walking
api-summary
splice
locking
diff --git a/Documentation/filesystems/path-walking.txt b/Documentation/filesystems/path-walking.rst
similarity index 91%
rename from Documentation/filesystems/path-walking.txt
rename to Documentation/filesystems/path-walking.rst
index 9b8930f589d9..941e5e0e0eed 100644
--- a/Documentation/filesystems/path-walking.txt
+++ b/Documentation/filesystems/path-walking.rst
@@ -1,3 +1,6 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====================================
Path walking and name lookup locking
====================================
@@ -64,6 +67,7 @@ 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;
@@ -118,45 +122,45 @@ 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.
+lookup can be retried::
- 1 2 3
- +---+ +---+ +---+
-hlist-->| N-+->| N-+->| N-+->
-head <--+-P |<-+-P |<-+-P |
- +---+ +---+ +---+
+ 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.
+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 |
- +---+
+ 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:
+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 |
- +---+
+ 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
@@ -210,7 +214,7 @@ 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
+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
@@ -219,14 +223,14 @@ 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.
+.. [#] 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
+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.
+ladder of snapshots to walk down::
proc 101
@@ -240,7 +244,7 @@ ladder of snapshots to walk down.
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.
+the context of proc101::
dentry 0
+---------------------+ rcu-walk begins here, we note d_seq, check the
@@ -288,6 +292,7 @@ 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
@@ -315,6 +320,7 @@ The detailed design for rcu-walk is like this:
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
@@ -345,12 +351,14 @@ 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
+::
+
+ 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
diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
index 8f7d25acf326..55a4bc87f664 100644
--- a/Documentation/filesystems/porting.rst
+++ b/Documentation/filesystems/porting.rst
@@ -404,7 +404,7 @@ the callback. It used to be necessary to clean it there, but not anymore
vfs now tries to do path walking in "rcu-walk mode", which avoids
atomic operations and scalability hazards on dentries and inodes (see
-Documentation/filesystems/path-walking.txt). d_hash and d_compare changes
+Documentation/filesystems/path-walking.rst). d_hash and d_compare changes
(above) are examples of the changes required to support this. For more complex
filesystem callbacks, the vfs drops out of rcu-walk mode before the fs call, so
no changes are required to the filesystem. However, this is costly and loses
diff --git a/fs/dcache.c b/fs/dcache.c
index cf8d5893bd0e..ab050262d0e6 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2191,7 +2191,7 @@ static inline bool d_same_name(const struct dentry *dentry,
*
* __d_lookup_rcu is the dcache lookup function for rcu-walk name
* resolution (store-free path walking) design described in
- * Documentation/filesystems/path-walking.txt.
+ * Documentation/filesystems/path-walking.rst.
*
* This is not to be used outside core vfs.
*
@@ -2239,7 +2239,7 @@ struct dentry *__d_lookup_rcu(const struct dentry *parent,
* false-negative result. d_lookup() protects against concurrent
* renames using rename_lock seqlock.
*
- * See Documentation/filesystems/path-walking.txt for more details.
+ * See Documentation/filesystems/path-walking.rst for more details.
*/
hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
unsigned seq;
@@ -2362,7 +2362,7 @@ struct dentry *__d_lookup(const struct dentry *parent, const struct qstr *name)
* false-negative result. d_lookup() protects against concurrent
* renames using rename_lock seqlock.
*
- * See Documentation/filesystems/path-walking.txt for more details.
+ * See Documentation/filesystems/path-walking.rst for more details.
*/
rcu_read_lock();
diff --git a/fs/namei.c b/fs/namei.c
index d1b53fea83d8..ae68041be7b6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -645,7 +645,7 @@ static bool legitimize_root(struct nameidata *nd)
/*
* Path walking has 2 modes, rcu-walk and ref-walk (see
- * Documentation/filesystems/path-walking.txt). In situations when we can't
+ * Documentation/filesystems/path-walking.rst). In situations when we can't
* continue in RCU mode, we attempt to drop out of rcu-walk mode and grab
* normal reference counts on dentries and vfsmounts to transition to ref-walk
* mode. Refcounts are grabbed at the last known good point before rcu-walk
--
2.25.2
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