Re: [PATCH 1/3] percpu: use ZERO_SIZE_PTR / ZERO_OR_NULL_PTR

[Christoph Lameter <cl@linux.com>]

On Mon, 30 Jan 2012, Tejun Heo wrote:

> Hello, Christoph.
>
> On Mon, Jan 30, 2012 at 11:52:23AM -0600, Christoph Lameter wrote:
> > We have two possibilities now:
> >
> > 1. We say that the value returned from the per cpu allocator is an opaque
> > value.
> >
> > 	This means that we have to remove the NULL check from the free
> > 	function. And audit the kernel code for all occurrences where
> > 	a per cpu pointer value of NULL is assumed to mean that no per
> > 	cpu allocation has occurred.
>
> No, NULL is never gonna be a valid return from any allocator including
> percpu.  Percpu allocator doesn't and will never do so.

How do you prevent the percpu allocator from returning NULL? I thought the
per cpu offsets can wrap around?

> > 2. We say that there are special values for the per cpu pointers (NULL,
> > 	ZERO_SIZE_PTR)
> >
> > 	Then we would have to guarantee that the per cpu allocator never
> > 	returns those values.
> >
> > 	Plus then the ZERO_SIZE_PTR patch will be fine.
> >
> > 	The danger exist of these values being passed as
> > 	parameters to functions that do not support them (per_cpu_ptr
> > 	etc). Those would need VM_BUG_ONs or some other checks to detect
> > 	potential problems.
>
> I'm saying we don't have this for ZERO_SIZE_PTR in any meaningful way
> at this point.  If somebody wants to implement it properly, please
> feel free to, but simply applying ZERO_SIZE_PTR without other changes
> doesn't make any sense.

We have no clean notion of how a percpu pointer needs to be handled. Both
ways of handling things have drawbacks.


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