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Date: Sat, 15 Sep 2007 01:39:13 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: menage@google.com (Paul Menage)
Cc: viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Add a refcount check in dput()
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On Mon, 10 Sep 2007 15:13:19 -0700 (PDT) menage@google.com (Paul Menage) wrote:

> Add a BUG_ON() to check for passing an unreferenced dentry to dput().
> 
> This is analogous to the similar check in dget(), and will make
> reference-counting bugs in filesystems more immediately obvious. (I
> just spent a while debugging an oops that turned out to be due to
> broken fs reference counting.)
> 
> Signed-off-by: Paul Menage <menage@google.com>
> 
> ---
>  fs/dcache.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> Index: container-2.6.23-rc3-mm1/fs/dcache.c
> ===================================================================
> --- container-2.6.23-rc3-mm1.orig/fs/dcache.c
> +++ container-2.6.23-rc3-mm1/fs/dcache.c
> @@ -176,6 +176,7 @@ void dput(struct dentry *dentry)
>  	if (!dentry)
>  		return;
>  
> +	BUG_ON(!atomic_read(&dentry->d_count));
>  repeat:
>  	if (atomic_read(&dentry->d_count) == 1)
>  		might_sleep();

eek, much too aggressive.

I added this:

--- a/fs/dcache.c~add-a-refcount-check-in-dput
+++ a/fs/dcache.c
@@ -176,6 +176,7 @@ void dput(struct dentry *dentry)
 	if (!dentry)
 		return;
 
+	WARN_ON_ONCE(!atomic_read(&dentry->d_count));
 repeat:
 	if (atomic_read(&dentry->d_count) == 1)
 		might_sleep();
_

to the -mm-only-debugging-patches section of the -mm tree.