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

[Waiman Long]

On 05/05/2015 11:25 AM, Raghavendra K T wrote:
> 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 ?

__TICKET_LOCK_INC is currently ((unsigned short)2), not ((unsigned 
int)2). That makes a difference.

Cheers,
Longman
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