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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