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Subject: Re: [PATCH 3/3] kref: Remove the memory barriers
From: Peter Zijlstra <peterz@infradead.org>
To: Ming Lei <tom.leiming@gmail.com>
Cc: gregkh@suse.de, akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        ostrikov@nvidia.com, adobriyan@gmail.com, eric.dumazet@gmail.com,
        mingo@elte.hu, Oliver Neukum <oneukum@suse.de>
Date: Mon, 12 Dec 2011 11:12:28 +0100
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On Mon, 2011-12-12 at 17:57 +0800, Ming Lei wrote:
> On Mon, Dec 12, 2011 at 4:54 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Mon, 2011-12-12 at 11:48 +0800, Ming Lei wrote:
> >> For kref, maybe it is still multiple operations done on one cpu vs them
> >> being visible on another, but seems a bit implicit, see the common kref
> >> usage below:
> >>
> >> CPU0                            CPU1
> >>                                         A:kref_init(&obj->ref)
> >
> > how does CPU0 get a ref to obj?
> 
> Suppose open/close/read/.. context is run on CPU0, and driver .probe/.release
> context(hotplug context) is run on CPU1. There are a few examples on
> usb driver(eg. usb-skeleton.c, ...)

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.