Re: [PATCH] x86/spinlocks: Fix regression in spinlock contention detection

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 05/05/2015 07:33 PM, Tahsin Erdogan wrote:
> The conversion to signed happens with types shorter than int (__ticket_t
> is either u8 or u16).
>
> By changing Raghavendra's program to use unsigned short int, you can see
> the problem:
>
> ================
> #include <stdio.h>
>
> #define LOCK_INC 2
>
> int main()
> {
>          unsigned short int head = 32700, tail=2;
>
>          if ((tail - head) > LOCK_INC)
>                  printf(" tail - head > LOCK_INC \n");
>          else
>                  printf(" tail - head < LOCK_INC \n");
>
>          return 0;
> }
>
> ================
> gcc -g -o t main.c
> ./t
>   tail - head < LOCK_INC
>
> However, having just unsigned int returns the opposite result (unsigned
> int head = 32700, tail=2;)
>

Interestingly,

#include <stdio.h>

//#define LOCK_INC ((unsigned int)2) // case 1
#define LOCK_INC 2 //case 2

int main()
{
	unsigned short int head = 32700, tail=2;

	if ((tail - head) > LOCK_INC)
		printf(" tail - head > LOCK_INC \n");
	else
		printf(" tail - head < LOCK_INC \n");

	return 0;
}

case 1 works here (PeterZ's stricter version)

case 2 gives tail - head < LOCK_INC

But is it not that we have case 1 we are looking here ?