VFS: Remove dentry->d_lock locking from shrink_dcache_for_umount_subtree()
Locks of the dcache_lock were replaced by locks of dentry->d_lock in commits
such as:
2304450783dfde7b0b94ae234edd0dbffa865073
2fd6b7f50797f2e993eea59e0a0b8c6399c811dc
as part of the RCU-based pathwalk changes, despite the fact that the caller
(shrink_dcache_for_umount()) notes in the banner comment the reasons that
d_lock is not necessary in these functions:
/*
* destroy the dentries attached to a superblock on unmounting
* - we don't need to use dentry->d_lock because:
* - the superblock is detached from all mountings and open files, so the
* dentry trees will not be rearranged by the VFS
* - s_umount is write-locked, so the memory pressure shrinker will ignore
* any dentries belonging to this superblock that it comes across
* - the filesystem itself is no longer permitted to rearrange the dentries
* in this superblock
*/
So remove these locks. If the locks are actually necessary, then this banner
comment should be altered instead.
The hash table chains are protected by 1-bit locks in the hash table heads, so
those shouldn't be a problem.
Note that to make this work, __d_drop() has to be split so that the RCUwalk
barrier can be avoided. This causes problems otherwise as it has an assertion
that dentry->d_lock is locked - but there is no need for that as no one else
can be trying to access this dentry, except to step over it (and that should
be handled by d_free(), I think).
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Nick Piggin <npiggin@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/dcache.c b/fs/dcache.c
index 7559057..9df8b86 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -301,6 +301,27 @@
return parent;
}
+/*
+ * Unhash a dentry without inserting an RCU walk barrier or checking that
+ * dentry->d_lock is locked. The caller must take care of that, if
+ * appropriate.
+ */
+static void __d_shrink(struct dentry *dentry)
+{
+ if (!d_unhashed(dentry)) {
+ struct hlist_bl_head *b;
+ if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
+ b = &dentry->d_sb->s_anon;
+ else
+ b = d_hash(dentry->d_parent, dentry->d_name.hash);
+
+ hlist_bl_lock(b);
+ __hlist_bl_del(&dentry->d_hash);
+ dentry->d_hash.pprev = NULL;
+ hlist_bl_unlock(b);
+ }
+}
+
/**
* d_drop - drop a dentry
* @dentry: dentry to drop
@@ -319,17 +340,7 @@
void __d_drop(struct dentry *dentry)
{
if (!d_unhashed(dentry)) {
- struct hlist_bl_head *b;
- if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
- b = &dentry->d_sb->s_anon;
- else
- b = d_hash(dentry->d_parent, dentry->d_name.hash);
-
- hlist_bl_lock(b);
- __hlist_bl_del(&dentry->d_hash);
- dentry->d_hash.pprev = NULL;
- hlist_bl_unlock(b);
-
+ __d_shrink(dentry);
dentry_rcuwalk_barrier(dentry);
}
}
@@ -832,10 +843,8 @@
BUG_ON(!IS_ROOT(dentry));
/* detach this root from the system */
- spin_lock(&dentry->d_lock);
dentry_lru_del(dentry);
- __d_drop(dentry);
- spin_unlock(&dentry->d_lock);
+ __d_shrink(dentry);
for (;;) {
/* descend to the first leaf in the current subtree */
@@ -844,16 +853,11 @@
/* this is a branch with children - detach all of them
* from the system in one go */
- spin_lock(&dentry->d_lock);
list_for_each_entry(loop, &dentry->d_subdirs,
d_u.d_child) {
- spin_lock_nested(&loop->d_lock,
- DENTRY_D_LOCK_NESTED);
dentry_lru_del(loop);
- __d_drop(loop);
- spin_unlock(&loop->d_lock);
+ __d_shrink(loop);
}
- spin_unlock(&dentry->d_lock);
/* move to the first child */
dentry = list_entry(dentry->d_subdirs.next,
@@ -885,10 +889,8 @@
list_del(&dentry->d_u.d_child);
} else {
parent = dentry->d_parent;
- spin_lock(&parent->d_lock);
parent->d_count--;
list_del(&dentry->d_u.d_child);
- spin_unlock(&parent->d_lock);
}
inode = dentry->d_inode;
@@ -935,9 +937,7 @@
dentry = sb->s_root;
sb->s_root = NULL;
- spin_lock(&dentry->d_lock);
dentry->d_count--;
- spin_unlock(&dentry->d_lock);
shrink_dcache_for_umount_subtree(dentry);
while (!hlist_bl_empty(&sb->s_anon)) {