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

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Date:	Mon, 15 Nov 2010 11:38:33 -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>,
	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 02:05 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.

No, we depend on if/else with constant arguments doing the right thing
all over the kernel.  It is always preferable to use it instead of
#ifdef where possible, so that the two branches of code are always
subjected to compiler checking, even if they're not being used.

>> +
>> +}
>> #else
>> -# define UNLOCK_LOCK_PREFIX
>> +static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
>> +{
>> +	barrier();
>> +	lock->tickets.head++;
>> +	barrier();
> The second barrier() is not needed.

Agreed.  It gets removed in a later patch.

    J
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