[RFC PATCH 18/25] adjust calling conventions of lock_for_kill(), fold __dentry_kill() into dentry_kill()

[Al Viro <viro@zeniv.linux.org.uk>]

Pull dropping ->d_lock on lock_for_kill() failure into lock_for_kill() itself.
That reduces dentry_kill() to
	if (!lock_for_kill(dentry))
		return NULL;
	return __dentry_kill(dentry);
at which point it's easier to move that if (...) into the beginning of __dentry_kill()
itself and rename it into dentry_kill().

Document the new calling conventions of lock_for_kill().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/dcache.c | 99 +++++++++++++++++++++++++++--------------------------
 1 file changed, 50 insertions(+), 49 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 6822e8bfc6af..125f280fe6ee 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -691,11 +691,60 @@ static inline void dentry_unlist(struct dentry *dentry)
 	}
 }
 
-static struct dentry *__dentry_kill(struct dentry *dentry)
+/*
+ * Prepare locking environment for killing a dentry.
+ * Called under dentry->d_lock.  To proceed with eviction of a positive
+ * dentry we need to get ->i_lock of dentry inode as well and that
+ * needs to be done carefully - ->i_lock nests outside of ->d_lock,
+ * so we might need to drop and regain the latter.  We use rcu_read_lock()
+ * to keep the RCU read-side critical area contiguous, so dentry and
+ * inode won't get freed under us, but dentry state might've changed
+ * while its ->d_lock had not been held - it might end up getting
+ * killed, becoming busy, negative, etc.
+ *
+ * If dentry is busy (or busy dying, or already dead), unlock dentry
+ * and return false.  Otherwise, return true and have that dentry's
+ * inode (if any) locked in addition to dentry itself.
+ */
+static bool lock_for_kill(struct dentry *dentry)
+{
+	struct inode *inode = dentry->d_inode;
+
+	if (unlikely(dentry->d_lockref.count)) {
+		spin_unlock(&dentry->d_lock);
+		return false;
+	}
+
+	if (!inode || likely(spin_trylock(&inode->i_lock)))
+		return true;
+
+	rcu_read_lock();
+	do {
+		spin_unlock(&dentry->d_lock);
+		spin_lock(&inode->i_lock);
+		spin_lock(&dentry->d_lock);
+		if (likely(inode == dentry->d_inode))
+			break;
+		spin_unlock(&inode->i_lock);
+		inode = dentry->d_inode;
+	} while (inode);
+	rcu_read_unlock();
+	if (likely(!dentry->d_lockref.count))
+		return true;
+	if (inode)
+		spin_unlock(&inode->i_lock);
+	spin_unlock(&dentry->d_lock);
+	return false;
+}
+
+static struct dentry *dentry_kill(struct dentry *dentry)
 {
 	struct dentry *parent = NULL;
 	bool can_free = true;
 
+	if (unlikely(!lock_for_kill(dentry)))
+		return NULL;
+
 	/*
 	 * The dentry is now unrecoverably dead to the world.
 	 */
@@ -742,54 +791,6 @@ static struct dentry *__dentry_kill(struct dentry *dentry)
 	return parent;
 }
 
-/*
- * Lock a dentry for feeding it to __dentry_kill().
- * Called under rcu_read_lock() and dentry->d_lock; the former
- * guarantees that nothing we access will be freed under us.
- * Note that dentry is *not* protected from concurrent dentry_kill(),
- * d_delete(), etc.
- *
- * Return false if dentry is busy.  Otherwise, return true and have
- * that dentry's inode locked.
- */
-
-static bool lock_for_kill(struct dentry *dentry)
-{
-	struct inode *inode = dentry->d_inode;
-
-	if (unlikely(dentry->d_lockref.count))
-		return false;
-
-	if (!inode || likely(spin_trylock(&inode->i_lock)))
-		return true;
-
-	rcu_read_lock();
-	do {
-		spin_unlock(&dentry->d_lock);
-		spin_lock(&inode->i_lock);
-		spin_lock(&dentry->d_lock);
-		if (likely(inode == dentry->d_inode))
-			break;
-		spin_unlock(&inode->i_lock);
-		inode = dentry->d_inode;
-	} while (inode);
-	rcu_read_unlock();
-	if (likely(!dentry->d_lockref.count))
-		return true;
-	if (inode)
-		spin_unlock(&inode->i_lock);
-	return false;
-}
-
-static struct dentry *dentry_kill(struct dentry *dentry)
-{
-	if (unlikely(!lock_for_kill(dentry))) {
-		spin_unlock(&dentry->d_lock);
-		return NULL;
-	}
-	return __dentry_kill(dentry);
-}
-
 /*
  * Decide if dentry is worth retaining.  Usually this is called with dentry
  * locked; if not locked, we are more limited and might not be able to tell
-- 
2.47.3