Re: [tip:locking/core] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

[Dan Lustig]

On 9/10/2021 6:04 AM, Boqun Feng wrote:
> On Fri, Sep 10, 2021 at 11:33:25AM +0200, Peter Zijlstra wrote:
>> On Fri, Sep 10, 2021 at 08:01:14AM +0800, Boqun Feng wrote:
>>> On Thu, Sep 09, 2021 at 01:03:18PM -0400, Dan Lustig wrote:
>>>> On 9/9/2021 9:35 AM, Will Deacon wrote:
>>>>> On Thu, Sep 09, 2021 at 09:25:30AM +0200, Peter Zijlstra wrote:
>>
>>>>>> The AMOSWAP is a RmW and as such matches the W from the RW->W fence,
>>>>>> similarly it marches the R from the R->RW fence, yielding an:
>>>>>>
>>>>>> 	RW->  W
>>>>>> 	    RmW
>>>>>> 	    R  ->RW
>>>>>>
>>>>>> ordering. It's the stores S and R that can be re-ordered, but not the
>>>>>> sections themselves (same on PowerPC and many others).
>>
>>>> I agree with Will here.  If the AMOSWAP above is actually implemented with
>>>> a RISC-V AMO, then the two critical sections will be separated as if RW,RW,
>>>> as Peter described.  If instead it's implemented using LR/SC, then RISC-V
>>>
>>> Just out of curiosity, in the following code, can the store S and load L
>>> be reordered?
>>>
>>> 	WRITE_ONCE(x, 1); // store S
>>> 	FENCE RW, W
>>>  	WRITE_ONCE(s.lock, 0); // unlock(s)
>>>  	AMOSWAP %0, 1, s.lock  // lock(s)
>>> 	FENCE R, RW
>>> 	r1 = READ_ONCE(y); // load L
>>>
>>> I think they can, because neither "FENCE RW, W" nor "FENCE R, RW" order
>>> them.
>>
>> I'm confused by your argument, per the above quoted section, those
>> fences and the AMO combine into a RW,RW ordering which is (as per the
>> later clarification) multi-copy-atomic, aka smp_mb().
>>
> 
> Right, my question is more about the reasoning about why fence rw,w +
> AMO + fence r,rw act as a fence rw,rw.

Is this a RISC-V question?  If so, it's as simple as:
1) S and anything earlier are ordered before the AMO by the first fence
2) L and anything later are ordered after the AMO by the second fence
3) 1 + 2 = S and anything earlier are ordered before L or anything later

Since RISC-V is multi-copy atomic, so 1+2 just naturally compose
transitively.

> Another related question, can
> fence rw,w + store + fence w,rw act as a fence rw,rw by the similar
> reasoning? IOW, will the two loads in the following be reordered?
> 
> 	r1 = READ_ONCE(x);
> 	FENCE RW, W
> 	WRITE_ONCE(z, 1);
> 	FENCE W, RW
> 	r2 = READ_ONCE(y);
> 
> again, this is more like a question out of curiosity, not that I find
> this pattern is useful.

Does FENCE W,RW appear in some actual use case?  But yes, if it does
appear, this sequence would also act as a FENCE RW,RW on RISC-V.

Dan

> Regards,
> Boqun
> 
>> As such, S and L are not allowed to be re-ordered in the given scenario.
>>
>>> Note that the reordering is allowed in LKMM, because unlock-lock
>>> only need to be as strong as RCtso.
>>
>> Risc-V is strictly stronger than required in this instance. Given the
>> current lock implementation. Daniel pointed out that if the atomic op
>> were LL/SC based instead of AMO it would end up being RCtso.
>>