linux-kernel - Re: [PATCH 1/3] percpu: use ZERO_SIZE_PTR / ZERO_OR_NULL

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Date:	Mon, 30 Jan 2012 11:35:29 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Tejun Heo <tj@...nel.org>
cc:	Dmitry Antipov <dmitry.antipov@...aro.org>,
	Rusty Russell <rusty@...tcorp.com.au>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, patches@...aro.org,
	linaro-dev@...ts.linaro.org
Subject: Re: [PATCH 1/3] percpu: use ZERO_SIZE_PTR / ZERO_OR_NULL_PTR

On Mon, 30 Jan 2012, Tejun Heo wrote:

> I'm pretty sure it never gives out NULL for a dynamic allocation.  The
> base might be mapped to zero but we're guaranteed to have some static
> percpu areas there and IIRC the percpu addresses aren't supposed to
> wrap.

True but there is a check for a NULL pointer on free. So a NULL pointer
currently has the semantics of being an unallocated per cpu structure.
If the allocator returns NULL by accident then we cannot free the per cpu
allocation anymore.
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