linux-kernel - Re: [PATCH 03/20] x86/ticketlock: Use C for __ticket_spin

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Date:	Sat, 13 Nov 2010 23:34:06 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	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 03/20] x86/ticketlock: Use C for __ticket_spin_unlock

On 11/13/2010 11:05 AM, Américo Wang wrote:
> On Wed, Nov 03, 2010 at 10:59:44AM -0400, Jeremy Fitzhardinge wrote:
>>   * On PPro SMP or if we are using OOSTORE, we use a locked operation to unlock
>>   * (PPro errata 66, 92)
>>   */
>> -# define UNLOCK_LOCK_PREFIX LOCK_PREFIX
>> +static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
>> +{
>> +	if (sizeof(lock->tickets.head) == sizeof(u8))
>> +		asm (LOCK_PREFIX "incb %0"
>> +		     : "+m" (lock->tickets.head) : : "memory");
>> +	else
>> +		asm (LOCK_PREFIX "incw %0"
>> +		     : "+m" (lock->tickets.head) : : "memory");
>
> This 'if/else' really should be done with #ifdef, even though
> the compiler may be smart enough to remove it.

That's a sure path to bitrot.

Paolo
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