RE: [PATCH v2] fix overflow in idle exit_latency

[David Laight <David.Laight@ACULAB.COM>]

From: Bo Ye 
> Sent: 19 December 2023 03:15
> 
> From: C Cheng <C.Cheng@mediatek.com>
> 
> In detail:
> 
> In C language, when you perform a multiplication operation, if
> both operands are of int type, the multiplication operation is
> performed on the int type, and then the result is converted to
> the target type. This means that if the product of int type
> multiplication exceeds the range that int type can represent,
>  an overflow will occur even if you store the result in a
> variable of int64_t type.
> 
> For a multiplication of two int values, it is better to use
> mul_u32_u32() rather than s->exit_latency_ns = s->exit_latency *
> NSEC_PER_USEC to avoid potential overflow happenning.
> 
> Signed-off-by: C Cheng <C.Cheng@mediatek.com>
> Signed-off-by: Bo Ye <bo.ye@mediatek.com>
> ---
> Change in v2:
> -remove Change-ID
> -correct patch author name
> -replace multiplication of two int values with mul_u32_u32
> -refine commit message
> ---
>  drivers/cpuidle/driver.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cpuidle/driver.c b/drivers/cpuidle/driver.c
> index d9cda7f6ccb9..cf5873cc45dc 100644
> --- a/drivers/cpuidle/driver.c
> +++ b/drivers/cpuidle/driver.c
> @@ -16,6 +16,7 @@
>  #include <linux/cpumask.h>
>  #include <linux/tick.h>
>  #include <linux/cpu.h>
> +#include <linux/math64.h>
> 
>  #include "cpuidle.h"
> 
> @@ -187,7 +188,7 @@ static void __cpuidle_driver_init(struct cpuidle_driver *drv)
>  			s->target_residency = div_u64(s->target_residency_ns, NSEC_PER_USEC);
> 
>  		if (s->exit_latency > 0)
> -			s->exit_latency_ns = s->exit_latency * NSEC_PER_USEC;
> +			s->exit_latency_ns = mul_u32_u32(s->exit_latency, NSEC_PER_USEC);

Just force either side of the multiply to a 64bit unsigned type.
The compiler will then DTRT which is likely to be better code than
whatever mul_u32_u32() generates.

In any case is the 'exit_latency' ever going to be greater than 4 seconds?
In which case the 32bit multiply will never overflow.

	David

>  		else if (s->exit_latency_ns < 0)
>  			s->exit_latency_ns =  0;
>  		else
> --
> 2.18.0
> 

-
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