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authorAl Viro <viro@zeniv.linux.org.uk>2018-03-10 23:15:52 -0500
committerAl Viro <viro@zeniv.linux.org.uk>2018-03-29 15:07:49 -0400
commit076515fc926793e162fc6525bed1679ef2bbf269 (patch)
treeafadea0e756147f4631df6b1bbef1c0e670b9494 /fs/dcache.c
parenta7498968338da9b928f5d8054acc8be6ed2bc14c (diff)
downloadlinux-076515fc926793e162fc6525bed1679ef2bbf269.tar.bz2
make non-exchanging __d_move() copy ->d_parent rather than swap them
Currently d_move(from, to) does the following: * name/parent of from <- old name/parent of to, from hashed there * to is unhashed * name of to is preserved * if from used to be detached, to gets detached * if from used to be attached, parent of to <- old parent of from. That's both user-visibly bogus and complicates reasoning a lot. Much saner semantics would be * name/parent of from <- name/parent of to, from hashed there. * to is unhashed * name/parent of to is unchanged. The price, of course, is that old parent of from might lose a reference. However, * all potentially cross-directory callers of d_move() have both parents pinned directly; typically, dentries themselves are grabbed only after we have grabbed and locked both parents. IOW, the decrement of old parent's refcount in case of d_move() won't reach zero. * __d_move() from d_splice_alias() is done to detached alias. No refcount decrements in that case * __d_move() from __d_unalias() *can* get the refcount to zero. So let's grab a reference to alias' old parent before calling __d_unalias() and dput() it after we'd dropped rename_lock. That does make d_splice_alias() potentially blocking. However, it has no callers in non-sleepable contexts (and the case where we'd grown that dget/dput pair is _very_ rare, so performance is not an issue). Another thing that needs adjustment is unlocking in the end of __d_move(); folded it in. And cleaned the remnants of bogus ordering from the "lock them in the beginning" counterpart - it's never been right and now (well, for 7 years now) we have that thing always serialized on rename_lock anyway. Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'fs/dcache.c')
-rw-r--r--fs/dcache.c93
1 files changed, 30 insertions, 63 deletions
diff --git a/fs/dcache.c b/fs/dcache.c
index c870343f904f..0a58038091f2 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -67,9 +67,7 @@
* dentry->d_lock
*
* If no ancestor relationship:
- * if (dentry1 < dentry2)
- * dentry1->d_lock
- * dentry2->d_lock
+ * arbitrary, since it's serialized on rename_lock
*/
int sysctl_vfs_cache_pressure __read_mostly = 100;
EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);
@@ -2777,9 +2775,6 @@ static void copy_name(struct dentry *dentry, struct dentry *target)
static void dentry_lock_for_move(struct dentry *dentry, struct dentry *target)
{
- /*
- * XXXX: do we really need to take target->d_lock?
- */
if (IS_ROOT(dentry) || dentry->d_parent == target->d_parent)
spin_lock(&target->d_parent->d_lock);
else {
@@ -2793,40 +2788,11 @@ static void dentry_lock_for_move(struct dentry *dentry, struct dentry *target)
DENTRY_D_LOCK_NESTED);
}
}
- if (target < dentry) {
- spin_lock_nested(&target->d_lock, 2);
- spin_lock_nested(&dentry->d_lock, 3);
- } else {
- spin_lock_nested(&dentry->d_lock, 2);
- spin_lock_nested(&target->d_lock, 3);
- }
-}
-
-static void dentry_unlock_for_move(struct dentry *dentry, struct dentry *target)
-{
- if (target->d_parent != dentry->d_parent)
- spin_unlock(&dentry->d_parent->d_lock);
- if (target->d_parent != target)
- spin_unlock(&target->d_parent->d_lock);
- spin_unlock(&target->d_lock);
- spin_unlock(&dentry->d_lock);
+ spin_lock_nested(&dentry->d_lock, 2);
+ spin_lock_nested(&target->d_lock, 3);
}
/*
- * When switching names, the actual string doesn't strictly have to
- * be preserved in the target - because we're dropping the target
- * anyway. As such, we can just do a simple memcpy() to copy over
- * the new name before we switch, unless we are going to rehash
- * it. Note that if we *do* unhash the target, we are not allowed
- * to rehash it without giving it a new name/hash key - whether
- * we swap or overwrite the names here, resulting name won't match
- * the reality in filesystem; it's only there for d_path() purposes.
- * Note that all of this is happening under rename_lock, so the
- * any hash lookup seeing it in the middle of manipulations will
- * be discarded anyway. So we do not care what happens to the hash
- * key in that case.
- */
-/*
* __d_move - move a dentry
* @dentry: entry to move
* @target: new dentry
@@ -2840,6 +2806,7 @@ static void dentry_unlock_for_move(struct dentry *dentry, struct dentry *target)
static void __d_move(struct dentry *dentry, struct dentry *target,
bool exchange)
{
+ struct dentry *old_parent;
struct inode *dir = NULL;
unsigned n;
if (!dentry->d_inode)
@@ -2858,49 +2825,47 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
write_seqcount_begin(&dentry->d_seq);
write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);
+ old_parent = dentry->d_parent;
+
/* unhash both */
if (!d_unhashed(dentry))
___d_drop(dentry);
if (!d_unhashed(target))
___d_drop(target);
- /* Switch the names.. */
- if (exchange)
- swap_names(dentry, target);
- else
+ /* ... and switch them in the tree */
+ dentry->d_parent = target->d_parent;
+ if (!exchange) {
copy_name(dentry, target);
-
- /* rehash in new place(s) */
- __d_rehash(dentry);
- if (exchange)
- __d_rehash(target);
- else
target->d_hash.pprev = NULL;
-
- /* ... and switch them in the tree */
- if (IS_ROOT(dentry)) {
- /* splicing a tree */
- dentry->d_flags |= DCACHE_RCUACCESS;
- dentry->d_parent = target->d_parent;
- target->d_parent = target;
- list_del_init(&target->d_child);
- list_move(&dentry->d_child, &dentry->d_parent->d_subdirs);
+ dentry->d_parent->d_lockref.count++;
+ if (dentry == old_parent)
+ dentry->d_flags |= DCACHE_RCUACCESS;
+ else
+ WARN_ON(!--old_parent->d_lockref.count);
} else {
- /* swapping two dentries */
- swap(dentry->d_parent, target->d_parent);
+ target->d_parent = old_parent;
+ swap_names(dentry, target);
list_move(&target->d_child, &target->d_parent->d_subdirs);
- list_move(&dentry->d_child, &dentry->d_parent->d_subdirs);
- if (exchange)
- fsnotify_update_flags(target);
- fsnotify_update_flags(dentry);
+ __d_rehash(target);
+ fsnotify_update_flags(target);
}
+ list_move(&dentry->d_child, &dentry->d_parent->d_subdirs);
+ __d_rehash(dentry);
+ fsnotify_update_flags(dentry);
write_seqcount_end(&target->d_seq);
write_seqcount_end(&dentry->d_seq);
if (dir)
end_dir_add(dir, n);
- dentry_unlock_for_move(dentry, target);
+
+ if (dentry->d_parent != old_parent)
+ spin_unlock(&dentry->d_parent->d_lock);
+ if (dentry != old_parent)
+ spin_unlock(&old_parent->d_lock);
+ spin_unlock(&target->d_lock);
+ spin_unlock(&dentry->d_lock);
}
/*
@@ -3048,12 +3013,14 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
inode->i_sb->s_type->name,
inode->i_sb->s_id);
} else if (!IS_ROOT(new)) {
+ struct dentry *old_parent = dget(new->d_parent);
int err = __d_unalias(inode, dentry, new);
write_sequnlock(&rename_lock);
if (err) {
dput(new);
new = ERR_PTR(err);
}
+ dput(old_parent);
} else {
__d_move(new, dentry, false);
write_sequnlock(&rename_lock);