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

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Date:	Tue, 5 May 2015 08:28:55 -0700
From:	Tahsin Erdogan <tahsin@...gle.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	Waiman.Long@...com, borntraeger@...ibm.com, oleg@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/spinlocks: Fix regression in spinlock contention detection

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

This works because it is casting to unsigned int. If you change it to
unsigned short int, it becomes consistent with case 2.

On Tue, May 5, 2015 at 8:25 AM, Raghavendra K T
<raghavendra.kt@...ux.vnet.ibm.com> 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 ?
>
>
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