lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Date:	Thu, 14 Dec 2006 08:56:44 +0100
From:	Eric Dumazet <>
To:	Andrew Morton <>
Cc:	Greg KH <>,
	Venkatesh Pallipadi <>,
	Arjan <>,
	linux-kernel <>,
	"Eric W. Biederman" <>
Subject: Re: kref refcnt and false positives

Andrew Morton a écrit :
> On Wed, 13 Dec 2006 16:12:46 -0800
> Greg KH <> wrote:
>>> Original comment seemed to indicate that this conditional thing was
>>> performance related. Is it really? If not, we should consider the below patch.
>> Yes, it's a performance gain and I don't see how this patch would change
>> the above warning.
> I suspect it's a false optimisation.
> int kref_put(struct kref *kref, void (*release)(struct kref *kref))
> {
> 	WARN_ON(release == NULL);
> 	WARN_ON(release == (void (*)(struct kref *))kfree);
> 	/*
> 	 * if current count is one, we are the last user and can release object
> 	 * right now, avoiding an atomic operation on 'refcount'
> 	 */
>  	if ((atomic_read(&kref->refcount) == 1) ||
> 	    (atomic_dec_and_test(&kref->refcount))) {
> 		release(kref);
> 		return 1;
> 	}
> 	return 0;
> }
> The only time we avoid the atomic_dec_and_test() is when the object is
> about to be freed.  ie: once in its entire lifetime.  And freeing the
> object is part of an expensive (and rare) operation anyway.
> otoh, we've gone and added a test-n-branch to the common case: those cases
> where the object will not be freed.

I agree this 'optimization' is not "good" (I was the guy who suggested it )

After Eric Biederman message ( I remember 
adding some stat counters and telling Greg to not put the patch in because 
kref_put() was mostly called with refcount=1. But the patch did its way. I 
*did* ask Greg to revert it, but cannot find this mail archived somewhere...

But I believe Venkatesh problem comes from its release() function : It is 
supposed to free the object.
If not, it should properly setup it so that further uses are OK.

ie doing in release(kref)
atomic_set(&kref->count, 0);

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

Powered by blists - more mailing lists