Re: [PATCH 2/3] percpu counter: cast this_cpu_sub() adjustment

[Greg Thelen]

On Sun, Oct 27 2013, Greg Thelen wrote:

> this_cpu_sub() is implemented as negation and addition.
>
> This patch casts the adjustment to the counter type before negation to
> sign extend the adjustment.  This helps in cases where the counter
> type is wider than an unsigned adjustment.  An alternative to this
> patch is to declare such operations unsupported, but it seemed useful
> to avoid surprises.
>
> This patch specifically helps the following example:
>   unsigned int delta = 1
>   preempt_disable()
>   this_cpu_write(long_counter, 0)
>   this_cpu_sub(long_counter, delta)
>   preempt_enable()
>
> Before this change long_counter on a 64 bit machine ends with value
> 0xffffffff, rather than 0xffffffffffffffff.  This is because
> this_cpu_sub(pcp, delta) boils down to this_cpu_add(pcp, -delta),
> which is basically:
>   long_counter = 0 + 0xffffffff
>
> Also apply the same cast to:
>   __this_cpu_sub()
>   this_cpu_sub_return()
>   and __this_cpu_sub_return()
>
> All percpu_test.ko passes, especially the following cases which
> previously failed:
>
>   l -= ui_one;
>   __this_cpu_sub(long_counter, ui_one);
>   CHECK(l, long_counter, -1);
>
>   l -= ui_one;
>   this_cpu_sub(long_counter, ui_one);
>   CHECK(l, long_counter, -1);
>   CHECK(l, long_counter, 0xffffffffffffffff);
>
>   ul -= ui_one;
>   __this_cpu_sub(ulong_counter, ui_one);
>   CHECK(ul, ulong_counter, -1);
>   CHECK(ul, ulong_counter, 0xffffffffffffffff);
>
>   ul = this_cpu_sub_return(ulong_counter, ui_one);
>   CHECK(ul, ulong_counter, 2);
>
>   ul = __this_cpu_sub_return(ulong_counter, ui_one);
>   CHECK(ul, ulong_counter, 1);
>
> Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/percpu.h | 3 ++-
>  include/linux/percpu.h        | 8 ++++----
>  lib/percpu_test.c             | 2 +-
>  3 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
> index 0da5200..b3e18f8 100644
> --- a/arch/x86/include/asm/percpu.h
> +++ b/arch/x86/include/asm/percpu.h
> @@ -128,7 +128,8 @@ do {							\
>  do {									\
>  	typedef typeof(var) pao_T__;					\
>  	const int pao_ID__ = (__builtin_constant_p(val) &&		\
> -			      ((val) == 1 || (val) == -1)) ? (val) : 0;	\
> +			      ((val) == 1 || (val) == -1)) ?		\
> +				(int)(val) : 0;				\
>  	if (0) {							\
>  		pao_T__ pao_tmp__;					\
>  		pao_tmp__ = (val);					\
> diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> index cc88172..c74088a 100644
> --- a/include/linux/percpu.h
> +++ b/include/linux/percpu.h
> @@ -332,7 +332,7 @@ do {									\
>  #endif
>  
>  #ifndef this_cpu_sub
> -# define this_cpu_sub(pcp, val)		this_cpu_add((pcp), -(val))
> +# define this_cpu_sub(pcp, val)		this_cpu_add((pcp), -(typeof(pcp))(val))
>  #endif
>  
>  #ifndef this_cpu_inc
> @@ -418,7 +418,7 @@ do {									\
>  # define this_cpu_add_return(pcp, val)	__pcpu_size_call_return2(this_cpu_add_return_, pcp, val)
>  #endif
>  
> -#define this_cpu_sub_return(pcp, val)	this_cpu_add_return(pcp, -(val))
> +#define this_cpu_sub_return(pcp, val)	this_cpu_add_return(pcp, -(typeof(pcp))(val))
>  #define this_cpu_inc_return(pcp)	this_cpu_add_return(pcp, 1)
>  #define this_cpu_dec_return(pcp)	this_cpu_add_return(pcp, -1)
>  
> @@ -586,7 +586,7 @@ do {									\
>  #endif
>  
>  #ifndef __this_cpu_sub
> -# define __this_cpu_sub(pcp, val)	__this_cpu_add((pcp), -(val))
> +# define __this_cpu_sub(pcp, val)	__this_cpu_add((pcp), -(typeof(pcp))(val))
>  #endif
>  
>  #ifndef __this_cpu_inc
> @@ -668,7 +668,7 @@ do {									\
>  	__pcpu_size_call_return2(__this_cpu_add_return_, pcp, val)
>  #endif
>  
> -#define __this_cpu_sub_return(pcp, val)	__this_cpu_add_return(pcp, -(val))
> +#define __this_cpu_sub_return(pcp, val)	__this_cpu_add_return(pcp, -(typeof(pcp))(val))
>  #define __this_cpu_inc_return(pcp)	__this_cpu_add_return(pcp, 1)
>  #define __this_cpu_dec_return(pcp)	__this_cpu_add_return(pcp, -1)
>  
> diff --git a/lib/percpu_test.c b/lib/percpu_test.c
> index 1ebeb44..8ab4231 100644
> --- a/lib/percpu_test.c
> +++ b/lib/percpu_test.c
> @@ -118,7 +118,7 @@ static int __init percpu_test_init(void)
>  	CHECK(ul, ulong_counter, 2);
>  
>  	ul = __this_cpu_sub_return(ulong_counter, ui_one);
> -	CHECK(ul, ulong_counter, 0);
> +	CHECK(ul, ulong_counter, 1);
>  
>  	preempt_enable();

Oops.  This update to percpu_test.c doesn't belong in this patch.  It
should be moved to the earlier patch 1/3.  I'll repost the series with
this update and the suggested changes to patch titles.
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