Re: [PATCH] x86/spinlocks: Fix regression in spinlock contention detection
From: Raghavendra K T
Date: Tue May 05 2015 - 12:22:09 EST
On 05/05/2015 09:02 PM, Waiman Long wrote:
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.
aah missed that part :). That makes sense.
Good catch Tahsin.
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