Subject: Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock

From: Eric Dumazet <[email-masked]>
Date: Wed, 08 Jul 2009 00:44:44 +0200

Mathieu Desnoyers a écrit :
> * Peter Zijlstra ([email-masked]) wrote:
>> On Tue, 2009-07-07 at 17:44 +0200, Oleg Nesterov wrote:
>>> On 07/07, Mathieu Desnoyers wrote:
>>>> Actually, thinking about it more, to appropriately support x86, as well
>>>> as powerpc, arm and mips, we would need something like:
>>>>
>>>> read_lock_smp_mb()
>>>>
>>>> Which would be a read_lock with an included memory barrier.
>>> Then we need read_lock_irq_smp_mb, read_lock_irqsave__smp_mb, write_lock_xxx,
>>> otherwise it is not clear why only read_lock() has _smp_mb() version.
>>>
>>> The same for spin_lock_xxx...
>> At which time the smp_mb__{before,after}_{un,}lock become attractive
>> again.
>>
> 
> Then having a new __read_lock() (without acquire semantic) which would
> be required to be followed by a smp__mb_after_lock() would make sense. I
> think this would fit all of x86, powerpc, arm, mips without having to
> create tons of new primitives. Only "simpler" ones that clearly separate
> locking from memory barriers.
> 

Hmm... On x86, read_lock() is :

	lock subl $0x1,(%eax)
	jns   .Lok
	call	__read_lock_failed
.Lok:	ret

What would be __read_lock() ? I cant see how it could *not* use lock prefix
actually and or being cheaper...