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authorNick Piggin <npiggin@kernel.dk>2011-01-07 01:49:38 -0500
committerNick Piggin <npiggin@kernel.dk>2011-01-07 01:50:23 -0500
commitb5c84bf6f6fa3a7dfdcb556023a62953574b60ee (patch)
tree7a2c299a180713e21d5cb653cb933121adf53c31 /Documentation
parent949854d02455080d20cd3e1db28a3a18daf7599d (diff)
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 a15ee207b449..bdad6414dfa0 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -21,14 +21,14 @@ prototypes:
21 char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen); 21 char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
22 22
23locking rules: 23locking rules:
24 dcache_lock rename_lock ->d_lock may block 24 rename_lock ->d_lock may block
25d_revalidate: no no no yes 25d_revalidate: no no yes
26d_hash no no no no 26d_hash no no no
27d_compare: no yes no no 27d_compare: yes no no
28d_delete: yes no yes no 28d_delete: no yes no
29d_release: no no no yes 29d_release: no no yes
30d_iput: no no no yes 30d_iput: no no yes
31d_dname: no no no no 31d_dname: no no no
32 32
33--------------------------- inode_operations --------------------------- 33--------------------------- inode_operations ---------------------------
34prototypes: 34prototypes:
diff --git a/Documentation/filesystems/dentry-locking.txt b/Documentation/filesystems/dentry-locking.txt
index 79334ed5daa7..30b6a40f5650 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
31doesn't acquire the dcache_lock for this and rely on RCU to ensure 31doesn't acquire the dcache_lock for this and rely on RCU to ensure
32that the dentry has not been *freed*. 32that the dentry has not been *freed*.
33 33
34dcache_lock no longer exists, dentry locking is explained in fs/dcache.c
34 35
35Dcache locking details 36Dcache locking details
36====================== 37======================
@@ -50,14 +51,12 @@ Safe lock-free look-up of dcache hash table
50 51
51Dcache is a complex data structure with the hash table entries also 52Dcache is a complex data structure with the hash table entries also
52linked together in other lists. In 2.4 kernel, dcache_lock protected 53linked together in other lists. In 2.4 kernel, dcache_lock protected
53all the lists. We applied RCU only on hash chain walking. The rest of 54all the lists. RCU dentry hash walking works like this:
54the lists are still protected by dcache_lock. Some of the important
55changes are :
56 55
571. The deletion from hash chain is done using hlist_del_rcu() macro 561. The deletion from hash chain is done using hlist_del_rcu() macro
58 which doesn't initialize next pointer of the deleted dentry and 57 which doesn't initialize next pointer of the deleted dentry and
59 this allows us to walk safely lock-free while a deletion is 58 this allows us to walk safely lock-free while a deletion is
60 happening. 59 happening. This is a standard hlist_rcu iteration.
61 60
622. Insertion of a dentry into the hash table is done using 612. Insertion of a dentry into the hash table is done using
63 hlist_add_head_rcu() which take care of ordering the writes - the 62 hlist_add_head_rcu() which take care of ordering the writes - the
@@ -66,19 +65,18 @@ changes are :
66 which has since been replaced by hlist_for_each_entry_rcu(), while 65 which has since been replaced by hlist_for_each_entry_rcu(), while
67 walking the hash chain. The only requirement is that all 66 walking the hash chain. The only requirement is that all
68 initialization to the dentry must be done before 67 initialization to the dentry must be done before
69 hlist_add_head_rcu() since we don't have dcache_lock protection 68 hlist_add_head_rcu() since we don't have lock protection
70 while traversing the hash chain. This isn't different from the 69 while traversing the hash chain.
71 existing code. 70
72 713. The dentry looked up without holding locks cannot be returned for
733. The dentry looked up without holding dcache_lock by cannot be 72 walking if it is unhashed. It then may have a NULL d_inode or other
74 returned for walking if it is unhashed. It then may have a NULL 73 bogosity since RCU doesn't protect the other fields in the dentry. We
75 d_inode or other bogosity since RCU doesn't protect the other 74 therefore use a flag DCACHE_UNHASHED to indicate unhashed dentries
76 fields in the dentry. We therefore use a flag DCACHE_UNHASHED to 75 and use this in conjunction with a per-dentry lock (d_lock). Once
77 indicate unhashed dentries and use this in conjunction with a 76 looked up without locks, we acquire the per-dentry lock (d_lock) and
78 per-dentry lock (d_lock). Once looked up without the dcache_lock, 77 check if the dentry is unhashed. If so, the look-up is failed. If not,
79 we acquire the per-dentry lock (d_lock) and check if the dentry is 78 the reference count of the dentry is increased and the dentry is
80 unhashed. If so, the look-up is failed. If not, the reference count 79 returned.
81 of the dentry is increased and the dentry is returned.
82 80
834. Once a dentry is looked up, it must be ensured during the path walk 814. Once a dentry is looked up, it must be ensured during the path walk
84 for that component it doesn't go away. In pre-2.5.10 code, this was 82 for that component it doesn't go away. In pre-2.5.10 code, this was
@@ -86,10 +84,10 @@ changes are :
86 In some sense, dcache_rcu path walking looks like the pre-2.5.10 84 In some sense, dcache_rcu path walking looks like the pre-2.5.10
87 version. 85 version.
88 86
895. All dentry hash chain updates must take the dcache_lock as well as 875. All dentry hash chain updates must take the per-dentry lock (see
90 the per-dentry lock in that order. dput() does this to ensure that 88 fs/dcache.c). This excludes dput() to ensure that a dentry that has
91 a dentry that has just been looked up in another CPU doesn't get 89 been looked up concurrently does not get deleted before dget() can
92 deleted before dget() can be done on it. 90 take a ref.
93 91
946. There are several ways to do reference counting of RCU protected 926. There are several ways to do reference counting of RCU protected
95 objects. One such example is in ipv4 route cache where deferred 93 objects. One such example is in ipv4 route cache where deferred
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 9fd31940a8ef..1eb76959d096 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -216,7 +216,6 @@ had ->revalidate()) add calls in ->follow_link()/->readlink().
216->d_parent changes are not protected by BKL anymore. Read access is safe 216->d_parent changes are not protected by BKL anymore. Read access is safe
217if at least one of the following is true: 217if at least one of the following is true:
218 * filesystem has no cross-directory rename() 218 * filesystem has no cross-directory rename()
219 * dcache_lock is held
220 * we know that parent had been locked (e.g. we are looking at 219 * we know that parent had been locked (e.g. we are looking at
221->d_parent of ->lookup() argument). 220->d_parent of ->lookup() argument).
222 * we are called from ->rename(). 221 * we are called from ->rename().
@@ -340,3 +339,10 @@ look at examples of other filesystems) for guidance.
340 .d_hash() calling convention and locking rules are significantly 339 .d_hash() calling convention and locking rules are significantly
341changed. Read updated documentation in Documentation/filesystems/vfs.txt (and 340changed. Read updated documentation in Documentation/filesystems/vfs.txt (and
342look at examples of other filesystems) for guidance. 341look at examples of other filesystems) for guidance.
342
343---
344[mandatory]
345 dcache_lock is gone, replaced by fine grained locks. See fs/dcache.c
346for details of what locks to replace dcache_lock with in order to protect
347particular things. Most of the time, a filesystem only needs ->d_lock, which
348protects *all* the dcache state of a given dentry.