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

[Oliver Neukum]

Am Montag, 12. Dezember 2011, 10:57:31 schrieb Ming Lei:
> On Mon, Dec 12, 2011 at 4:54 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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

	Regards
		Oliver
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