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>

1 files changed, 24 insertions, 24 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 75590572ff7a..9df8b861e18e 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -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)) {