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>

1 files changed, 15 insertions, 12 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index a8f89765d602..195706374697 100644
--- 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);