[RFC][PATCH 08/10] fs: Do RCU versions for find_inode()

[Peter Zijlstra]

Now that i_count is a proper reference count, such that 0 means free
or freeing, and all .destroy_inode methods use RCU to free inodes, we
can trivially convert the inode hash to RCU and do RCU lookups.

So provide RCU variants of find_inode() and find_inode_fast(), in case
we do hit an inode with i_count==0, we fall back to the old code that
does a __wait_for_freeing_inode().

_However_ this makes the situation with using i_count() for decisions
far worse; those few that are in the evict/free path seem safe, the
rest is up for grabs.

If the rest of the sites were OK, they probably are no longer and need
help.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 fs/inode.c |  104 ++++++++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 76 insertions(+), 28 deletions(-)

--- a/fs/inode.c
+++ b/fs/inode.c
@@ -471,7 +471,7 @@ void __insert_inode_hash(struct inode *i
 
 	spin_lock(&inode_hash_lock);
 	spin_lock(&inode->i_lock);
-	hlist_add_head(&inode->i_hash, b);
+	hlist_add_head_rcu(&inode->i_hash, b);
 	spin_unlock(&inode->i_lock);
 	spin_unlock(&inode_hash_lock);
 }
@@ -487,7 +487,7 @@ void __remove_inode_hash(struct inode *i
 {
 	spin_lock(&inode_hash_lock);
 	spin_lock(&inode->i_lock);
-	hlist_del_init(&inode->i_hash);
+	hlist_del_init_rcu(&inode->i_hash);
 	spin_unlock(&inode->i_lock);
 	spin_unlock(&inode_hash_lock);
 }
@@ -777,15 +777,15 @@ long prune_icache_sb(struct super_block
 }
 
 static void __wait_on_freeing_inode(struct inode *inode);
-/*
- * Called with the inode lock held.
- */
-static struct inode *find_inode(struct super_block *sb,
+
+static struct inode *__find_inode(struct super_block *sb,
 				struct hlist_head *head,
 				int (*test)(struct inode *, void *),
 				void *data)
 {
-	struct inode *inode = NULL;
+	struct inode *inode;
+
+	lockdep_assert_held(&inode_hash_lock);
 
 repeat:
 	hlist_for_each_entry(inode, head, i_hash) {
@@ -805,14 +805,44 @@ static struct inode *find_inode(struct s
 	return NULL;
 }
 
+static struct inode *find_inode(struct super_block *sb,
+				struct hlist_head *head,
+				int (*test)(struct inode *, void *),
+				void *data)
+{
+	struct inode *inode;
+
+	rcu_read_lock();
+	hlist_for_each_entry_rcu(inode, head, i_hash) {
+		if (inode->i_sb != sb)
+			continue;
+		if (!test(inode, data))
+			continue;
+		if (atomic_inc_not_zero(&inode->i_count))
+			goto out_unlock;
+		goto slow;
+	}
+	inode = NULL;
+out_unlock:
+	rcu_read_unlock();
+	return inode;
+
+slow:
+	rcu_read_unlock();
+	spin_lock(&inode_hash_lock);
+	inode = __find_inode(sb, head, test, data);
+	spin_unlock(&inode_hash_lock);
+	return inode;
+}
+
 /*
- * find_inode_fast is the fast path version of find_inode, see the comment at
+ * __find_inode_fast is the fast path version of __find_inode, see the comment at
  * iget_locked for details.
  */
-static struct inode *find_inode_fast(struct super_block *sb,
-				struct hlist_head *head, unsigned long ino)
+static struct inode *__find_inode_fast(struct super_block *sb,
+		struct hlist_head *head, unsigned long ino)
 {
-	struct inode *inode = NULL;
+	struct inode *inode;
 
 	lockdep_assert_held(&inode_hash_lock);
 
@@ -834,6 +864,34 @@ static struct inode *find_inode_fast(str
 	return NULL;
 }
 
+static struct inode *find_inode_fast(struct super_block *sb,
+		struct hlist_head *head, unsigned long ino)
+{
+	struct inode *inode;
+
+	rcu_read_lock();
+	hlist_for_each_entry_rcu(inode, head, i_hash) {
+		if (inode->i_ino != ino)
+			continue;
+		if (inode->i_sb != sb)
+			continue;
+		if (atomic_inc_not_zero(&inode->i_count))
+			goto out_unlock;
+		goto slow;
+	}
+	inode = NULL;
+out_unlock:
+	rcu_read_unlock();
+	return inode;
+
+slow:
+	rcu_read_unlock();
+	spin_lock(&inode_hash_lock);
+	inode = __find_inode_fast(sb, head, ino);
+	spin_unlock(&inode_hash_lock);
+	return inode;
+}
+
 /*
  * Each cpu owns a range of LAST_INO_BATCH numbers.
  * 'shared_last_ino' is dirtied only once out of LAST_INO_BATCH allocations,
@@ -1026,10 +1084,7 @@ struct inode *iget5_locked(struct super_
 	struct hlist_head *head = inode_hashtable + hash(sb, hashval);
 	struct inode *inode;
 again:
-	spin_lock(&inode_hash_lock);
 	inode = find_inode(sb, head, test, data);
-	spin_unlock(&inode_hash_lock);
-
 	if (inode) {
 		wait_on_inode(inode);
 		if (unlikely(inode_unhashed(inode))) {
@@ -1045,14 +1100,14 @@ struct inode *iget5_locked(struct super_
 
 		spin_lock(&inode_hash_lock);
 		/* We released the lock, so.. */
-		old = find_inode(sb, head, test, data);
+		old = __find_inode(sb, head, test, data);
 		if (!old) {
 			if (set(inode, data))
 				goto set_failed;
 
 			spin_lock(&inode->i_lock);
 			inode->i_state = I_NEW;
-			hlist_add_head(&inode->i_hash, head);
+			hlist_add_head_rcu(&inode->i_hash, head);
 			spin_unlock(&inode->i_lock);
 			inode_sb_list_add(inode);
 			spin_unlock(&inode_hash_lock);
@@ -1104,9 +1159,7 @@ struct inode *iget_locked(struct super_b
 	struct hlist_head *head = inode_hashtable + hash(sb, ino);
 	struct inode *inode;
 again:
-	spin_lock(&inode_hash_lock);
 	inode = find_inode_fast(sb, head, ino);
-	spin_unlock(&inode_hash_lock);
 	if (inode) {
 		wait_on_inode(inode);
 		if (unlikely(inode_unhashed(inode))) {
@@ -1122,12 +1175,12 @@ struct inode *iget_locked(struct super_b
 
 		spin_lock(&inode_hash_lock);
 		/* We released the lock, so.. */
-		old = find_inode_fast(sb, head, ino);
+		old = __find_inode_fast(sb, head, ino);
 		if (!old) {
 			inode->i_ino = ino;
 			spin_lock(&inode->i_lock);
 			inode->i_state = I_NEW;
-			hlist_add_head(&inode->i_hash, head);
+			hlist_add_head_rcu(&inode->i_hash, head);
 			spin_unlock(&inode->i_lock);
 			inode_sb_list_add(inode);
 			spin_unlock(&inode_hash_lock);
@@ -1258,9 +1311,7 @@ struct inode *ilookup5_nowait(struct sup
 	struct hlist_head *head = inode_hashtable + hash(sb, hashval);
 	struct inode *inode;
 
-	spin_lock(&inode_hash_lock);
 	inode = find_inode(sb, head, test, data);
-	spin_unlock(&inode_hash_lock);
 
 	return inode;
 }
@@ -1313,10 +1364,7 @@ struct inode *ilookup(struct super_block
 	struct hlist_head *head = inode_hashtable + hash(sb, ino);
 	struct inode *inode;
 again:
-	spin_lock(&inode_hash_lock);
 	inode = find_inode_fast(sb, head, ino);
-	spin_unlock(&inode_hash_lock);
-
 	if (inode) {
 		wait_on_inode(inode);
 		if (unlikely(inode_unhashed(inode))) {
@@ -1345,7 +1393,7 @@ EXPORT_SYMBOL(ilookup);
  * the inode_hash_lock spinlock held.
  *
  * This is a even more generalized version of ilookup5() when the
- * function must never block --- find_inode() can block in
+ * function must never block --- __find_inode() can block in
  * __wait_on_freeing_inode() --- or when the caller can not increment
  * the reference count because the resulting iput() might cause an
  * inode eviction.  The tradeoff is that the @match funtion must be
@@ -1402,7 +1450,7 @@ int insert_inode_locked(struct inode *in
 		if (likely(!old)) {
 			spin_lock(&inode->i_lock);
 			inode->i_state |= I_NEW;
-			hlist_add_head(&inode->i_hash, head);
+			hlist_add_head_rcu(&inode->i_hash, head);
 			spin_unlock(&inode->i_lock);
 			spin_unlock(&inode_hash_lock);
 			return 0;
@@ -1445,7 +1493,7 @@ int insert_inode_locked4(struct inode *i
 		if (likely(!old)) {
 			spin_lock(&inode->i_lock);
 			inode->i_state |= I_NEW;
-			hlist_add_head(&inode->i_hash, head);
+			hlist_add_head_rcu(&inode->i_hash, head);
 			spin_unlock(&inode->i_lock);
 			spin_unlock(&inode_hash_lock);
 			return 0;