linux-kernel - [CFT] unify dentry_iput() and dentry_unlink

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Date:	Mon, 30 May 2016 01:27:54 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: [CFT] unify dentry_iput() and dentry_unlink_inode()

[bringing back an old tangent from a thread back in February]

There is a lot of duplication between dentry_unlink_inode() and dentry_iput().
The only real difference is that dentry_unlink_inode() bumps ->d_seq and
dentry_iput() doesn't.  The argument of the latter is known to have been
unhashed, so anybody who might've found it in RCU lookup would already be
doomed to a ->d_seq mismatch.  And we want to avoid pointless smp_rmb() there.
 
This patch makes dentry_unlink_inode() bump ->d_seq only for hashed dentries.
It's safe (d_delete() calls that sucker only if we are holding the only
reference to dentry, so rehash is not going to happen) and it allows
to use dentry_unlink_inode() in __dentry_kill() and get rid of dentry_iput().
    
The interesting question here is profiling; it *is* a hot path, and extra
conditional jumps in there might or might not be painful.
    
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---

diff --git a/fs/dcache.c b/fs/dcache.c
index f9c63c1..fe7cde2 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -335,44 +335,21 @@ static inline void dentry_rcuwalk_invalidate(struct dentry *dentry)
 
 /*
  * Release the dentry's inode, using the filesystem
- * d_iput() operation if defined. Dentry has no refcount
- * and is unhashed.
- */
-static void dentry_iput(struct dentry * dentry)
-	__releases(dentry->d_lock)
-	__releases(dentry->d_inode->i_lock)
-{
-	struct inode *inode = dentry->d_inode;
-	if (inode) {
-		__d_clear_type_and_inode(dentry);
-		hlist_del_init(&dentry->d_u.d_alias);
-		spin_unlock(&dentry->d_lock);
-		spin_unlock(&inode->i_lock);
-		if (!inode->i_nlink)
-			fsnotify_inoderemove(inode);
-		if (dentry->d_op && dentry->d_op->d_iput)
-			dentry->d_op->d_iput(dentry, inode);
-		else
-			iput(inode);
-	} else {
-		spin_unlock(&dentry->d_lock);
-	}
-}
-
-/*
- * Release the dentry's inode, using the filesystem
- * d_iput() operation if defined. dentry remains in-use.
+ * d_iput() operation if defined.
  */
 static void dentry_unlink_inode(struct dentry * dentry)
 	__releases(dentry->d_lock)
 	__releases(dentry->d_inode->i_lock)
 {
 	struct inode *inode = dentry->d_inode;
+	bool hashed = !d_unhashed(dentry);
 
-	raw_write_seqcount_begin(&dentry->d_seq);
+	if (hashed)
+		raw_write_seqcount_begin(&dentry->d_seq);
 	__d_clear_type_and_inode(dentry);
 	hlist_del_init(&dentry->d_u.d_alias);
-	raw_write_seqcount_end(&dentry->d_seq);
+	if (hashed)
+		raw_write_seqcount_end(&dentry->d_seq);
 	spin_unlock(&dentry->d_lock);
 	spin_unlock(&inode->i_lock);
 	if (!inode->i_nlink)
@@ -540,12 +517,10 @@ static void __dentry_kill(struct dentry *dentry)
 	dentry->d_flags |= DCACHE_DENTRY_KILLED;
 	if (parent)
 		spin_unlock(&parent->d_lock);
-	dentry_iput(dentry);
-	/*
-	 * dentry_iput drops the locks, at which point nobody (except
-	 * transient RCU lookups) can reach this dentry.
-	 */
-	BUG_ON(dentry->d_lockref.count > 0);
+	if (dentry->d_inode)
+		dentry_unlink_inode(dentry);
+	else
+		spin_unlock(&dentry->d_lock);
 	this_cpu_dec(nr_dentry);
 	if (dentry->d_op && dentry->d_op->d_release)
 		dentry->d_op->d_release(dentry);