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

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Date:	Tue, 5 May 2015 07:10:19 -0700
From:	Tahsin Erdogan <tahsin@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	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

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;)

gcc -g -o t main.c
./t
 tail - head > LOCK_INC

On Tue, May 5, 2015 at 3:58 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, May 05, 2015 at 04:08:22PM +0530, Raghavendra K T wrote:
>> ================
>> #include <stdio.h>
>>
>> #define LOCK_INC 2
>
> Note that to be completely identical to the kernel code you should've
> written:
>
> #define LOCK_INC        ((unsigned int)2)
>
> as per:
>
> #define TICKET_LOCK_INC ((__ticket_t)__TICKET_LOCK_INC)
>
>
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