fs: dcache reduce prune_one_dentry locking

prune_one_dentry can avoid quite a bit of locking in the common case where
ancestors have an elevated refcount. Alternatively, we could have gone the
other way and made fewer trylocks in the case where d_count goes to zero, but
is probably less common.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

diff --git a/fs/dcache.c b/fs/dcache.c
index a8f89765d6027c3c6c882c947582882c53238d07..1957063746977c7bc1e72446223589e772890a40 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -582,26 +582,29 @@ static void prune_one_dentry(struct dentry *dentry, struct dentry *parent)
         * Prune ancestors.
         */
        while (dentry) {
-               spin_lock(&dcache_inode_lock);
-again:
+relock:
                spin_lock(&dentry->d_lock);
+               if (dentry->d_count > 1) {
+                       dentry->d_count--;
+                       spin_unlock(&dentry->d_lock);
+                       return;
+               }
+               if (!spin_trylock(&dcache_inode_lock)) {
+relock2:
+                       spin_unlock(&dentry->d_lock);
+                       cpu_relax();
+                       goto relock;
+               }
+
                if (IS_ROOT(dentry))
                        parent = NULL;
                else
                        parent = dentry->d_parent;
                if (parent && !spin_trylock(&parent->d_lock)) {
-                       spin_unlock(&dentry->d_lock);
-                       goto again;
-               }
-               dentry->d_count--;
-               if (dentry->d_count) {
-                       if (parent)
-                               spin_unlock(&parent->d_lock);
-                       spin_unlock(&dentry->d_lock);
                        spin_unlock(&dcache_inode_lock);
-                       return;
+                       goto relock2;
                }
-
+               dentry->d_count--;
                dentry_lru_del(dentry);
                __d_drop(dentry);
                dentry = d_kill(dentry, parent);