linux-kernel - Re: [PATCH] tools/memory-model: Make ppo a subrelation of po

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Date:   Tue, 24 Jan 2023 13:54:14 +0100
From:   Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     paulmck@...nel.org, parri.andrea@...il.com, will@...nel.org,
        peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com, dlustig@...dia.com, joel@...lfernandes.org,
        urezki@...il.com, quic_neeraju@...cinc.com, frederic@...nel.org,
        linux-kernel@...r.kernel.org, viktor@...-sws.org
Subject: Re: [PATCH] tools/memory-model: Make ppo a subrelation of po



On 1/23/2023 9:25 PM, Alan Stern wrote:
> On Mon, Jan 23, 2023 at 07:25:48PM +0100, Jonas Oberhauser wrote:
>> Alright, after some synchronization in the other parts of this thread I am
>> beginning to prepare the next iteration of the patch.
>>
>> On 1/19/2023 4:13 AM, Alan Stern wrote:
>>> On Wed, Jan 18, 2023 at 10:38:11PM +0100, Jonas Oberhauser wrote:
>>>> On 1/18/2023 8:52 PM, Alan Stern wrote:
>>>>> On Tue, Jan 17, 2023 at 08:31:59PM +0100, Jonas Oberhauser wrote:
>>>>>> -	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
>>>>>> -	([M] ; po ; [UL] ; (co | po) ; [LKW] ;
>>>>>> -		fencerel(After-unlock-lock) ; [M])
>>>>>> +	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M])
>>>>> Shouldn't the po case of (co | po) remain intact here?
>>>> You can leave it here, but it is already covered by two other parts: the
>>>> ordering given through ppo/hb is covered by the po-unlock-lock-po & int in
>>>> ppo, and the ordering given through pb is covered by its inclusion in
>>>> strong-order.
>>> What about the ordering given through
>>> A-cumul(strong-fence)/cumul-fence/prop/hb?  I suppose that might be
>>> superseded by pb as well, but it seems odd not to have it in hb.
>> How should we resolve this?
>> My current favorite (compromise :D) solution would be to
>> 1. still eliminate both po and co cases from first definition of
>> strong-fence which is used in ppo,
>> 2. define a relation equal to the strong-order in this patch (with po|rf)
> Wouldn't it need to have po|co?  Consider:
>
> 	Wx=1	Rx=1		Ry=1		Rz=1
> 		lock(s)		lock(s)		lock(s)
> 		unlock(s)	unlock(s)	unlock(s)
> 		Wy=1		Wz=1		smp_mb__after_unlock_lock
> 						Rx=0
>
> With the co term this is forbidden.  With only the rf term it is
> allowed, because po-unlock-lock-po isn't A-cumulative.
No, but unlock() is ( 
https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/tree/tools/memory-model/lock.cat?h=dev.2023.01.19a#n67 
). So you get

   Rx=0 ->overwrite Wx=1  ->rfe Rx1 ->po-rel  T1:unlock(s) ->rfe 
T2:lock(s) ->po-unlock-lock-po;after ... fence;po Rx=0
which is
   Rx=0          ->prop ;                           
po-unlock-lock-po;after ... fence;po Rx=0

Are you ok going forward like this then?

If not, I might prefer to redefine po-unlock-lock-po into something that 
works for all its use cases if possible. I think
|

   po ; [UL] ; (po|co?;rf) ; [LKR] ; po

|might be such a definition but haven't fully thought about it.

best wishes, jonas