fs: dcache reduce d_parent locking

Use RCU to simplify locking in dget_parent.

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

diff --git a/fs/dcache.c b/fs/dcache.c
index b4d2e28eef5b1e0ee9a78e8d9b58566312300f5d..a8f89765d6027c3c6c882c947582882c53238d07 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -446,24 +446,27 @@ struct dentry *dget_parent(struct dentry *dentry)
        struct dentry *ret;
 
 repeat:
-       spin_lock(&dentry->d_lock);
+       /*
+        * Don't need rcu_dereference because we re-check it was correct under
+        * the lock.
+        */
+       rcu_read_lock();
        ret = dentry->d_parent;
-       if (!ret)
-               goto out;
-       if (dentry == ret) {
-               ret->d_count++;
+       if (!ret) {
+               rcu_read_unlock();
                goto out;
        }
-       if (!spin_trylock(&ret->d_lock)) {
-               spin_unlock(&dentry->d_lock);
-               cpu_relax();
+       spin_lock(&ret->d_lock);
+       if (unlikely(ret != dentry->d_parent)) {
+               spin_unlock(&ret->d_lock);
+               rcu_read_unlock();
                goto repeat;
        }
+       rcu_read_unlock();
        BUG_ON(!ret->d_count);
        ret->d_count++;
        spin_unlock(&ret->d_lock);
 out:
-       spin_unlock(&dentry->d_lock);
        return ret;
 }
 EXPORT_SYMBOL(dget_parent);