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

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Date:	Tue, 05 May 2015 11:32:32 -0400
From:	Waiman Long <waiman.long@...com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	Tahsin Erdogan <tahsin@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	borntraeger@...ibm.com, oleg@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/spinlocks: Fix regression in spinlock contention
 detection

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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