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Commit c6627c60 authored by David Howells's avatar David Howells Committed by Al Viro
Browse files

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:

	23044507
	2fd6b7f5

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: default avatarDavid Howells <dhowells@redhat.com>
Cc: Nick Piggin <npiggin@kernel.dk>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 35f40ef0

fs/dcache.c

+24 −24
Original line number Diff line number Diff line
@@ -301,6 +301,27 @@ static struct dentry *d_kill(struct dentry *dentry, struct dentry *parent) 
	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 @@ static struct dentry *d_kill(struct dentry *dentry, struct dentry *parent) 
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 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry) 
	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 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry) 


			/* 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 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry) 
				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 @@ void shrink_dcache_for_umount(struct super_block *sb) 


	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)) {