3 files changed, 65 insertions, 49 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 2d244227999d..96655f4f4574 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1786,7 +1786,7 @@ static noinline enum slow_d_compare slow_dentry_cmp(
  * without taking d_lock and checking d_seq sequence count against @seq
  * returned here.
  *
- * A refcount may be taken on the found dentry with the __d_rcu_to_refcount
+ * A refcount may be taken on the found dentry with the d_rcu_to_refcount
  * function.
  *
  * Alternatively, __d_lookup_rcu may be called again to look up the child of
diff --git a/fs/namei.c b/fs/namei.c
index 7720fbd5277b..2c30c84d4ea1 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -494,6 +494,50 @@ static inline void unlock_rcu_walk(void)
         br_read_unlock(&vfsmount_lock);
 }
 
+/*
+ * When we move over from the RCU domain to properly refcounted
+ * long-lived dentries, we need to check the sequence numbers
+ * we got before lookup very carefully.
+ *
+ * We cannot blindly increment a dentry refcount - even if it
+ * is not locked - if it is zero, because it may have gone
+ * through the final d_kill() logic already.
+ *
+ * So for a zero refcount, we need to get the spinlock (which is
+ * safe even for a dead dentry because the de-allocation is
+ * RCU-delayed), and check the sequence count under the lock.
+ *
+ * Once we have checked the sequence count, we know it is live,
+ * and since we hold the spinlock it cannot die from under us.
+ *
+ * In contrast, if the reference count wasn't zero, we can just
+ * increment the lockref without having to take the spinlock.
+ * Even if the sequence number ends up being stale, we haven't
+ * gone through the final dput() and killed the dentry yet.
+ */
+static inline int d_rcu_to_refcount(struct dentry *dentry, seqcount_t *validate, unsigned seq)
+{
+        int gotref;
+
+        gotref = lockref_get_or_lock(&dentry->d_lockref);
+
+        /* Does the sequence number still match? */
+        if (read_seqcount_retry(validate, seq)) {
+                if (gotref)
+                        dput(dentry);
+                else
+                        spin_unlock(&dentry->d_lock);
+                return -ECHILD;
+        }
+
+        /* Get the ref now, if we couldn't get it originally */
+        if (!gotref) {
+                dentry->d_lockref.count++;
+                spin_unlock(&dentry->d_lock);
+        }
+        return 0;
+}
+
 /**
  * unlazy_walk - try to switch to ref-walk mode.
  * @nd: nameidata pathwalk data
@@ -518,29 +562,28 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry)
                                 nd->root.dentry != fs->root.dentry)
                         goto err_root;
         }
-        spin_lock(&parent->d_lock);
+
+        /*
+         * For a negative lookup, the lookup sequence point is the parents
+         * sequence point, and it only needs to revalidate the parent dentry.
+         *
+         * For a positive lookup, we need to move both the parent and the
+         * dentry from the RCU domain to be properly refcounted. And the
+         * sequence number in the dentry validates *both* dentry counters,
+         * since we checked the sequence number of the parent after we got
+         * the child sequence number. So we know the parent must still
+         * be valid if the child sequence number is still valid.
+         */
         if (!dentry) {
-                if (!__d_rcu_to_refcount(parent, nd->seq))
-                        goto err_parent;
+                if (d_rcu_to_refcount(parent, &parent->d_seq, nd->seq) < 0)
+                        goto err_root;
                 BUG_ON(nd->inode != parent->d_inode);
         } else {
-                if (dentry->d_parent != parent)
+                if (d_rcu_to_refcount(dentry, &dentry->d_seq, nd->seq) < 0)
+                        goto err_root;
+                if (d_rcu_to_refcount(parent, &dentry->d_seq, nd->seq) < 0)
                         goto err_parent;
-                spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
-                if (!__d_rcu_to_refcount(dentry, nd->seq))
-                        goto err_child;
-                /*
-                 * If the sequence check on the child dentry passed, then
-                 * the child has not been removed from its parent. This
-                 * means the parent dentry must be valid and able to take
-                 * a reference at this point.
-                 */
-                BUG_ON(!IS_ROOT(dentry) && dentry->d_parent != parent);
-                BUG_ON(!parent->d_lockref.count);
-                parent->d_lockref.count++;
-                spin_unlock(&dentry->d_lock);
         }
-        spin_unlock(&parent->d_lock);
         if (want_root) {
                 path_get(&nd->root);
                 spin_unlock(&fs->lock);
@@ -551,10 +594,8 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry)
         nd->flags &= ~LOOKUP_RCU;
         return 0;
 
-err_child:
-        spin_unlock(&dentry->d_lock);
 err_parent:
-        spin_unlock(&parent->d_lock);
+        dput(dentry);
 err_root:
         if (want_root)
                 spin_unlock(&fs->lock);
@@ -585,14 +626,11 @@ static int complete_walk(struct nameidata *nd)
                 nd->flags &= ~LOOKUP_RCU;
                 if (!(nd->flags & LOOKUP_ROOT))
                         nd->root.mnt = NULL;
-                spin_lock(&dentry->d_lock);
-                if (unlikely(!__d_rcu_to_refcount(dentry, nd->seq))) {
-                        spin_unlock(&dentry->d_lock);
+
+                if (d_rcu_to_refcount(dentry, &dentry->d_seq, nd->seq) < 0) {
                         unlock_rcu_walk();
                         return -ECHILD;
                 }
-                BUG_ON(nd->inode != dentry->d_inode);
-                spin_unlock(&dentry->d_lock);
                 mntget(nd->path.mnt);
                 unlock_rcu_walk();
         }
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index efdc94434c30..9169b91ea2d2 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -304,28 +304,6 @@ extern struct dentry *__d_lookup(const struct dentry *, const struct qstr *);
 extern struct dentry *__d_lookup_rcu(const struct dentry *parent,
                                 const struct qstr *name, unsigned *seq);
 
-/**
- * __d_rcu_to_refcount - take a refcount on dentry if sequence check is ok
- * @dentry: dentry to take a ref on
- * @seq: seqcount to verify against
- * Returns: 0 on failure, else 1.
- *
- * __d_rcu_to_refcount operates on a dentry,seq pair that was returned
- * by __d_lookup_rcu, to get a reference on an rcu-walk dentry.
- */
-static inline int __d_rcu_to_refcount(struct dentry *dentry, unsigned seq)
-{
-        int ret = 0;
-
-        assert_spin_locked(&dentry->d_lock);
-        if (!read_seqcount_retry(&dentry->d_seq, seq)) {
-                ret = 1;
-                dentry->d_lockref.count++;
-        }
-
-        return ret;
-}
-
 static inline unsigned d_count(const struct dentry *dentry)
 {
         return dentry->d_lockref.count;