> On Jul 12, 2019, at 6:46 PM, David Miller <davem@xxxxxxxxxxxxx> wrote:
>
> From: Qian Cai <cai@xxxxxx>
> Date: Fri, 12 Jul 2019 15:23:21 -0400
>
>> The commit d66acc39c7ce ("bitops: Optimise get_order()") introduced a
>> problem for the be2net driver as "rx_frag_size" could be a module
>> parameter that can be changed while loading the module.
>
> Why is this a problem?
Well, for example, if rx_frag_size was set to 8096 when loading the module, the kernel has already used the default value 2048 during compilation time.
>
>> That commit checks __builtin_constant_p() first in get_order() which
>> cause "adapter->big_page_size" to be assigned a value based on the
>> the default "rx_frag_size" value at the compilation time. It also
>> generate a compilation warning,
>
> rx_frag_size is not a constant, therefore the __builtin_constant_p()
> test should not pass.
>
> This explanation doesn't seem valid.
Actually, GCC would consider it a const with -O2 optimized level because it found that it was never modified and it does not understand it is a module parameter. Considering the following code.
# cat const.c
#include <stdio.h>
static int a = 1;
int main(void)
{
if (__builtin_constant_p(a))
printf("a is a const.\n");
return 0;
}
# gcc -O2 const.c -o const
# ./const
a is a const.
