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

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Date:	Mon, 12 Dec 2011 15:14:19 -0800
From:	Greg KH <greg@...ah.com>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	Ming Lei <tom.leiming@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, gregkh@...e.de,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	ostrikov@...dia.com, adobriyan@...il.com, eric.dumazet@...il.com,
	mingo@...e.hu
Subject: Re: [PATCH 3/3] kref: Remove the memory barriers

On Mon, Dec 12, 2011 at 11:56:47PM +0100, Oliver Neukum wrote:
> Am Montag, 12. Dezember 2011, 20:30:40 schrieb Greg KH:
> > On Mon, Dec 12, 2011 at 11:20:16AM +0100, Oliver Neukum wrote:
> > > Am Montag, 12. Dezember 2011, 10:57:31 schrieb Ming Lei:
> > > > On Mon, Dec 12, 2011 at 4:54 PM, Peter Zijlstra <peterz@...radead.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, ...)
> > > 
> > > USB generally relies on an implied barrier just as:
> > > 
> > >         /* we can register the device now, as it is ready */
> > >         retval = usb_register_dev(interface, &skel_class);
> > > 
> > > Generally reference counting cannot help you if kfree() is involved
> > 
> > So, Oliver, you don't have any objection to this patch removing the
> > barriers in kref, right?  Originally you added them, I just wanted to
> > make sure before I applied this.
> 
> I do not remember any more why I introduced this.
> 
> I guess I worried not about the increment, but the decrement.
> Which makes me wonder what happens if you don't intend
> to get the kref again, but need to make sure it is usually freed,
> like:
> 
> CPU A								CPU B
> 
> kref_get(p)
> start_io(p)
> 									[interrupt from IO]
> 									kref_put(p)
> 
> You need an ordering primitive between start_io() and kref_get()
> or the counter could go negative.

Really?  On an atomic variable?  I didn't think this was needed for
atomics to ensure this type of thing couldn't happen.

> I think I was worried about it missing.

I'd be worried as well, but I don't think that can really happen, or am
I totally wrong?

greg k-h
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