linux-kernel - Re: [PATCH 3/3] kref: Remove the memory barriers

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Date:	Mon, 12 Dec 2011 12:13:13 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, gregkh@...e.de,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	ostrikov@...dia.com, adobriyan@...il.com, mingo@...e.hu,
	Oliver Neukum <oneukum@...e.de>
Subject: Re: [PATCH 3/3] kref: Remove the memory barriers

Le lundi 12 décembre 2011 à 18:32 +0800, Ming Lei a écrit :
> On Mon, Dec 12, 2011 at 6:12 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > I don't know the driver model, and I don't plan to start learning it
> > now. But if what you said is possible its broken and no memory barriers
> > will fix it.
> 
> IMO, you don't need to learn it, and my example is very simple and common
> kref usage in device drivers, :-)
> 
> Could we only focus on it and see what is problem? and why won't memory
> barrier fix it?
> 
> Anyway, device drivers are the most consumers of kref...

Really, kref is not magical per itself.

Using kref (or any refcounting) doesnt avoid bugs.

In the example you gave, there is a clear bug :

CPU0                            CPU1
                                        A:kref_init(&obj->ref)

B:kref_get(&obj->ref)
C:access obj                    E:access obj
                                        F:kref_put(&ojb->ref)
D:kref_put(obj->ref)


because _nothing_ prevents CPU0 accessing obj right after F:

To solve this problem, you need another way to synchronize accesses.



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