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authorNick Piggin <npiggin@kernel.dk>2011-01-07 17:49:38 +1100
committerNick Piggin <npiggin@kernel.dk>2011-01-07 17:50:23 +1100
commitb5c84bf6f6fa3a7dfdcb556023a62953574b60ee (patch)
tree7a2c299a180713e21d5cb653cb933121adf53c31 /Documentation
parent949854d02455080d20cd3e1db28a3a18daf7599d (diff)
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fs: dcache remove dcache_lock
dcache_lock no longer protects anything. remove it. Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/filesystems/Locking16
-rw-r--r--Documentation/filesystems/dentry-locking.txt40
-rw-r--r--Documentation/filesystems/porting8
3 files changed, 34 insertions, 30 deletions
diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index a15ee207b44..bdad6414dfa 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -21,14 +21,14 @@ prototypes:
char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
locking rules:
- dcache_lock rename_lock ->d_lock may block
-d_revalidate: no no no yes
-d_hash no no no no
-d_compare: no yes no no
-d_delete: yes no yes no
-d_release: no no no yes
-d_iput: no no no yes
-d_dname: no no no no
+ rename_lock ->d_lock may block
+d_revalidate: no no yes
+d_hash no no no
+d_compare: yes no no
+d_delete: no yes no
+d_release: no no yes
+d_iput: no no yes
+d_dname: no no no
--------------------------- inode_operations ---------------------------
prototypes:
diff --git a/Documentation/filesystems/dentry-locking.txt b/Documentation/filesystems/dentry-locking.txt
index 79334ed5daa..30b6a40f565 100644
--- a/Documentation/filesystems/dentry-locking.txt
+++ b/Documentation/filesystems/dentry-locking.txt
@@ -31,6 +31,7 @@ significant change is the way d_lookup traverses the hash chain, it
doesn't acquire the dcache_lock for this and rely on RCU to ensure
that the dentry has not been *freed*.
+dcache_lock no longer exists, dentry locking is explained in fs/dcache.c
Dcache locking details
======================
@@ -50,14 +51,12 @@ Safe lock-free look-up of dcache hash table
Dcache is a complex data structure with the hash table entries also
linked together in other lists. In 2.4 kernel, dcache_lock protected
-all the lists. We applied RCU only on hash chain walking. The rest of
-the lists are still protected by dcache_lock. Some of the important
-changes are :
+all the lists. RCU dentry hash walking works like this:
1. The deletion from hash chain is done using hlist_del_rcu() macro
which doesn't initialize next pointer of the deleted dentry and
this allows us to walk safely lock-free while a deletion is
- happening.
+ happening. This is a standard hlist_rcu iteration.
2. Insertion of a dentry into the hash table is done using
hlist_add_head_rcu() which take care of ordering the writes - the
@@ -66,19 +65,18 @@ changes are :
which has since been replaced by hlist_for_each_entry_rcu(), while
walking the hash chain. The only requirement is that all
initialization to the dentry must be done before
- hlist_add_head_rcu() since we don't have dcache_lock protection
- while traversing the hash chain. This isn't different from the
- existing code.
-
-3. The dentry looked up without holding dcache_lock by cannot be
- returned for walking if it is unhashed. It then may have a NULL
- d_inode or other bogosity since RCU doesn't protect the other
- fields in the dentry. We therefore use a flag DCACHE_UNHASHED to
- indicate unhashed dentries and use this in conjunction with a
- per-dentry lock (d_lock). Once looked up without the dcache_lock,
- we acquire the per-dentry lock (d_lock) and check if the dentry is
- unhashed. If so, the look-up is failed. If not, the reference count
- of the dentry is increased and the dentry is returned.
+ hlist_add_head_rcu() since we don't have lock protection
+ while traversing the hash chain.
+
+3. The dentry looked up without holding locks cannot be returned for
+ walking if it is unhashed. It then may have a NULL d_inode or other
+ bogosity since RCU doesn't protect the other fields in the dentry. We
+ therefore use a flag DCACHE_UNHASHED to indicate unhashed dentries
+ and use this in conjunction with a per-dentry lock (d_lock). Once
+ looked up without locks, we acquire the per-dentry lock (d_lock) and
+ check if the dentry is unhashed. If so, the look-up is failed. If not,
+ the reference count of the dentry is increased and the dentry is
+ returned.
4. Once a dentry is looked up, it must be ensured during the path walk
for that component it doesn't go away. In pre-2.5.10 code, this was
@@ -86,10 +84,10 @@ changes are :
In some sense, dcache_rcu path walking looks like the pre-2.5.10
version.
-5. All dentry hash chain updates must take the dcache_lock as well as
- the per-dentry lock in that order. dput() does this to ensure that
- a dentry that has just been looked up in another CPU doesn't get
- deleted before dget() can be done on it.
+5. All dentry hash chain updates must take the per-dentry lock (see
+ fs/dcache.c). This excludes dput() to ensure that a dentry that has
+ been looked up concurrently does not get deleted before dget() can
+ take a ref.
6. There are several ways to do reference counting of RCU protected
objects. One such example is in ipv4 route cache where deferred
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 9fd31940a8e..1eb76959d09 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -216,7 +216,6 @@ had ->revalidate()) add calls in ->follow_link()/->readlink().
->d_parent changes are not protected by BKL anymore. Read access is safe
if at least one of the following is true:
* filesystem has no cross-directory rename()
- * dcache_lock is held
* we know that parent had been locked (e.g. we are looking at
->d_parent of ->lookup() argument).
* we are called from ->rename().
@@ -340,3 +339,10 @@ look at examples of other filesystems) for guidance.
.d_hash() calling convention and locking rules are significantly
changed. Read updated documentation in Documentation/filesystems/vfs.txt (and
look at examples of other filesystems) for guidance.
+
+---
+[mandatory]
+ dcache_lock is gone, replaced by fine grained locks. See fs/dcache.c
+for details of what locks to replace dcache_lock with in order to protect
+particular things. Most of the time, a filesystem only needs ->d_lock, which
+protects *all* the dcache state of a given dentry.