fs: dcache reduce d_parent locking

Use RCU to simplify locking in dget_parent.

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

1 files changed, 12 insertions, 9 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index b4d2e28eef5b..a8f89765d602 100644
--- 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);