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

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Message-ID: <4CD1A33E.6040105@goop.org>
Date:	Wed, 03 Nov 2010 14:00:30 -0400
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Eric Dumazet <eric.dumazet@...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 03/20] x86/ticketlock: Use C for __ticket_spin_unlock

On 11/03/2010 11:13 AM, Eric Dumazet wrote:
> Le mercredi 03 novembre 2010 à 10:59 -0400, Jeremy Fitzhardinge a
> écrit :
>> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
>>
>> If we don't need to use a locked inc for unlock, then implement it in C.
>>
>> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
>> ---
>>  arch/x86/include/asm/spinlock.h |   33 ++++++++++++++++++---------------
>>  1 files changed, 18 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
>> index 6711d36..082990a 100644
>> --- a/arch/x86/include/asm/spinlock.h
>> +++ b/arch/x86/include/asm/spinlock.h
>> @@ -33,9 +33,23 @@
>>   * 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");
>> +
>> +}
>>  #else
>> -# define UNLOCK_LOCK_PREFIX
>> +static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
>> +{
>> +	barrier();
> technically speaking, it should be : smp_wmb()

Perhaps.  In practise it won't make a difference because it is defined
as barrier() unless OOSTORE is defined, in which case we need to do a
locked increment anyway.

Thanks,
    J
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