linux-kernel - Re: [PATCH v2] fix __percpu annotation in asm-generic

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Date:   Wed, 4 Dec 2019 17:28:37 -0500
From:   Dennis Zhou <dennis@...nel.org>
To:     Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Cc:     Dennis Zhou <dennis@...nel.org>, linux-kernel@...r.kernel.org,
        Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Ben Dooks <ben.dooks@...ethink.co.uk>
Subject: Re: [PATCH v2] fix __percpu annotation in asm-generic

On Wed, Dec 04, 2019 at 02:06:23AM +0100, Luc Van Oostenryck wrote:
> The generic implementation of raw_cpu_generic_add_return() is:
> 
>         #define raw_cpu_generic_add_return(pcp, val)            \
>         ({                                                      \
>                 typeof(&(pcp)) __p = raw_cpu_ptr(&(pcp));       \
>                                                                 \
>                 *__p += val;                                    \
>                 *__p;                                           \
>         })
> 
> where the 'pcp' argument is a __percpu lvalue.
> There, the variable '__p' is declared as a __percpu pointer
> the type of the address of 'pcp') but:
> 1) the value assigned to it, the return value of raw_cpu_ptr(), is
>    a plain (__kernel) pointer, not a __percpu one.
> 2) this variable is dereferenced just after while a __percpu
>    pointer is implicitly __noderef.
> 
> So, fix the declaration of the 'pcp' variable to its correct type:
> the plain (non-percpu) pointer corresponding to pcp's address,
> using the fact that typeof() ignores the address space and the
> 'noderef' attribute of its agument.
> 
> Same for raw_cpu_generic_xchg(), raw_cpu_generic_cmpxchg() &
> raw_cpu_generic_cmpxchg_double().
> 
> This removes 209 warnings on ARM, 525 on ARM64, 220 on x86 &
> more than 2600 on ppc64 (all of them with the default config).
> 
> Cc: Dennis Zhou <dennis@...nel.org>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Reported-by: Ben Dooks <ben.dooks@...ethink.co.uk>
> Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@...il.com>
> ---
> 
> Change since v1:
> * use the fact that typeof() ignore the address space of its argument.
> 
>  include/asm-generic/percpu.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.h
> index c2de013b2cf4..35e4a53b83e6 100644
> --- a/include/asm-generic/percpu.h
> +++ b/include/asm-generic/percpu.h
> @@ -74,7 +74,7 @@ do {									\
>  
>  #define raw_cpu_generic_add_return(pcp, val)				\
>  ({									\
> -	typeof(&(pcp)) __p = raw_cpu_ptr(&(pcp));			\
> +	typeof(pcp) *__p = raw_cpu_ptr(&(pcp));				\
>  									\
>  	*__p += val;							\
>  	*__p;								\
> @@ -82,7 +82,7 @@ do {									\
>  
>  #define raw_cpu_generic_xchg(pcp, nval)					\
>  ({									\
> -	typeof(&(pcp)) __p = raw_cpu_ptr(&(pcp));			\
> +	typeof(pcp) *__p = raw_cpu_ptr(&(pcp));				\
>  	typeof(pcp) __ret;						\
>  	__ret = *__p;							\
>  	*__p = nval;							\
> @@ -91,7 +91,7 @@ do {									\
>  
>  #define raw_cpu_generic_cmpxchg(pcp, oval, nval)			\
>  ({									\
> -	typeof(&(pcp)) __p = raw_cpu_ptr(&(pcp));			\
> +	typeof(pcp) *__p = raw_cpu_ptr(&(pcp));				\
>  	typeof(pcp) __ret;						\
>  	__ret = *__p;							\
>  	if (__ret == (oval))						\
> @@ -101,8 +101,8 @@ do {									\
>  
>  #define raw_cpu_generic_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2) \
>  ({									\
> -	typeof(&(pcp1)) __p1 = raw_cpu_ptr(&(pcp1));			\
> -	typeof(&(pcp2)) __p2 = raw_cpu_ptr(&(pcp2));			\
> +	typeof(pcp1) *__p1 = raw_cpu_ptr(&(pcp1));			\
> +	typeof(pcp2) *__p2 = raw_cpu_ptr(&(pcp2));			\
>  	int __ret = 0;							\
>  	if (*__p1 == (oval1) && *__p2  == (oval2)) {			\
>  		*__p1 = nval1;						\
> -- 
> 2.24.0
> 

I've applied this for-5.6.

Thanks,
Dennis