[rfc][patch] fs: block_dump missing dentry locking

[Nick Piggin <npiggin@suse.de>]

I think the block_dump output in __mark_inode_dirty is missing dentry locking.
Surely the i_dentry list can change any time, so we may not even *get* a
dentry there. If we do get one by chance, then it would appear to be able to
go away or get renamed at any time...

I've just gone the very-safe route and used the dcache helper to find this,
and done the full refcounting/locking thing. Also just moved it to its
own function to reduce clutter a bit.

Is my thinking correct?

Thanks,
Nick
---
 fs/fs-writeback.c |   41 ++++++++++++++++++++++++-----------------
 1 file changed, 24 insertions(+), 17 deletions(-)

Index: linux-2.6/fs/fs-writeback.c
===================================================================
--- linux-2.6.orig/fs/fs-writeback.c
+++ linux-2.6/fs/fs-writeback.c
@@ -64,6 +64,28 @@ static void writeback_release(struct bac
 	clear_bit(BDI_pdflush, &bdi->state);
 }
 
+static noinline void block_dump___mark_inode_dirty(struct inode *inode)
+{
+	if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev")) {
+		struct dentry *dentry;
+		const char *name = "?";
+
+		dentry = d_find_alias(inode);
+		if (dentry) {
+			spin_lock(&dentry->d_lock);
+			name = (const char *) dentry->d_name.name;
+		}
+		printk(KERN_DEBUG
+		       "%s(%d): dirtied inode %lu (%s) on %s\n",
+		       current->comm, task_pid_nr(current), inode->i_ino,
+		       name, inode->i_sb->s_id);
+		if (dentry) {
+			spin_unlock(&dentry->d_lock);
+			dput(dentry);
+		}
+	}
+}
+
 /**
  *	__mark_inode_dirty -	internal function
  *	@inode: inode to mark
@@ -114,23 +136,8 @@ void __mark_inode_dirty(struct inode *in
 	if ((inode->i_state & flags) == flags)
 		return;
 
-	if (unlikely(block_dump)) {
-		struct dentry *dentry = NULL;
-		const char *name = "?";
-
-		if (!list_empty(&inode->i_dentry)) {
-			dentry = list_entry(inode->i_dentry.next,
-					    struct dentry, d_alias);
-			if (dentry && dentry->d_name.name)
-				name = (const char *) dentry->d_name.name;
-		}
-
-		if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev"))
-			printk(KERN_DEBUG
-			       "%s(%d): dirtied inode %lu (%s) on %s\n",
-			       current->comm, task_pid_nr(current), inode->i_ino,
-			       name, inode->i_sb->s_id);
-	}
+	if (unlikely(block_dump))
+		block_dump___mark_inode_dirty(inode);
 
 	spin_lock(&inode_lock);
 	if ((inode->i_state & flags) != flags) {