linux-kernel - Re: [PATCH 7/8] percpu: add __percpu sparse annotations to hw

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Date:	Tue, 26 Jan 2010 03:18:48 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, axboe@...nel.dk,
	rusty@...tcorp.com.au, akpm@...ux-foundation.org,
	ebiederm@...ssion.com, tytso@....edu, Trond.Myklebust@...app.com,
	aelder@....com, hch@...radead.org, viro@...iv.linux.org.uk,
	davem@...emloft.net, netdev@...r.kernel.org, x86@...nel.org,
	mingo@...hat.com, dan.j.williams@...el.com,
	borislav.petkov@....com, ying.huang@...el.com, lenb@...nel.org,
	neilb@...e.de, cl@...ux-foundation.org
Subject: Re: [PATCH 7/8] percpu: add __percpu sparse annotations to
	hw_breakpoint

On Tue, Jan 26, 2010 at 11:13:59AM +0900, Tejun Heo wrote:
> Hello,
> 
> On 01/26/2010 11:01 AM, Frederic Weisbecker wrote:
> > So, may be it considers you are applying the address space overriding
> > to the pointer to the type and not to the type itself.
> > 
> > Consider:
> > 
> > 	int __percpu i;
> > 
> > What you do above *might* be considered as if SHIFT_PERCPU_PTR
> > returns something of a type:
> > 
> > 	int * __percpu i;
> > 
> > So the pointer is in the normal address space, but its content is in
> > __percpu address space.
> > 
> > What if you do this:
> > 
> > 
> > #define SHIFT_PERCPU_PTR(__p, __offset)      ({                      \
> >       __verify_pcpu_ptr((__p));                                       \
> >       RELOC_HIDE((__p), (__offset)); \
> > })
> > 
> > #define per_cpu(var, cpu) \
> >       (typeof(var) __kernel __force)(*SHIFT_PERCPU_PTR(&(var), per_cpu_offset(cpu)))
> 
> arch/x86/kernel/cpu/common.c:1149:20: warning: cast to non-scalar
> arch/x86/kernel/cpu/common.c:1149:20: error: strange non-value function or array
>   CC      arch/x86/kernel/cpu/common.o
> arch/x86/kernel/cpu/common.c: In function 'cpu_init':
> arch/x86/kernel/cpu/common.c:1149: error: cast specifies array type
> 
> Can't cast that way.  :-(


What about this? It doesn't use direct cast to scalar but should
create a pointer type to kernel space datas:


#define kernel_space_t(var)	\
	(typeof(var)	__kernel __force)

#define SHIFT_PERCPU_PTR(__p, __offset) ({                              \
        __verify_pcpu_ptr((__p));                                       \
        RELOC_HIDE((typeof(*(kernel_space_t(var)) __kernel __force *)(__p), (__offset)); \
})

#define per_cpu(var, cpu) \
        (*SHIFT_PERCPU_PTR(&(var), per_cpu_offset(cpu)))

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