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make non-exchanging __d_move() copy ->d_parent rather than swap them

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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>
This commit is contained in:
Al Viro 2018-03-10 23:15:52 -05:00
parent a749896833
commit 076515fc92
1 changed files with 31 additions and 64 deletions
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95
fs/dcache.c
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@ -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 {
@ -2793,39 +2788,10 @@ 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);
}
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);
}
/*
* 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
@ -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
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 = target->d_parent;
if (!exchange) {
copy_name(dentry, target);
target->d_hash.pprev = NULL;
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);