linux-kernel - Re: [PATCH 01/20] x86/ticketlock: clean up types and accessors

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Date:	Mon, 15 Nov 2010 11:36:54 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Américo Wang <xiyou.wangcong@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jan Beulich <JBeulich@...ell.com>, Avi Kivity <avi@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 01/20] x86/ticketlock: clean up types and accessors

On 11/13/2010 01:57 AM, Américo Wang wrote:
> On Wed, Nov 03, 2010 at 10:59:42AM -0400, Jeremy Fitzhardinge wrote:
> ...
>> static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>> {
>> -	int inc = 0x00010000;
>> -	int tmp;
>> +	unsigned inc = 1 << TICKET_SHIFT;
>> +	unsigned tmp;
> Please don't use old implicit-int.

I don't mind much either way, but I don't think I've seen anyone worry
about this before.

>> -	return (((tmp >> TICKET_SHIFT) - tmp) & ((1 << TICKET_SHIFT) - 1)) > 1;
>> +	return ((tmp.tail - tmp.head) & TICKET_MASK) > 1;
>
> There is a type promotion here.

...

>> }
>>
>> #ifndef CONFIG_PARAVIRT_SPINLOCKS
>> diff --git a/arch/x86/include/asm/spinlock_types.h b/arch/x86/include/asm/spinlock_types.h
>> index dcb48b2..4582640 100644
>> --- a/arch/x86/include/asm/spinlock_types.h
>> +++ b/arch/x86/include/asm/spinlock_types.h
>> @@ -5,11 +5,27 @@
>> # error "please don't include this file directly"
>> #endif
>>
>> +#include <linux/types.h>
>> +
>> +#if (CONFIG_NR_CPUS < 256)
>> +typedef u8  __ticket_t;
>> +#else
>> +typedef u16 __ticket_t;
>> +#endif
>> +
>> +#define TICKET_SHIFT	(sizeof(__ticket_t) * 8)
>> +#define TICKET_MASK	((1 << TICKET_SHIFT) - 1)
>
> So here you may need to cast the result to __ticket_t.

OK.

    J
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