linux-kernel - Re: [PATCHv2 3/4] tools/memory-model: Define effect of Mb tags on RMWs in tools/...

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Date: Wed, 5 Jun 2024 21:56:31 +0200
From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: paulmck@...nel.org, stern@...land.harvard.edu, parri.andrea@...il.com,
 will@...nel.org, peterz@...radead.org, 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 Vafeiadis <viktor@...-sws.org>
Subject: Re: [PATCHv2 3/4] tools/memory-model: Define effect of Mb tags on
 RMWs in tools/...

Am 6/5/2024 um 6:28 AM schrieb Boqun Feng:
> On Tue, Jun 04, 2024 at 06:04:40PM +0200, Jonas Oberhauser wrote:
>> Herd7 transforms successful RMW with Mb tags by inserting smp_mb() fences
>> around them. We emulate this by considering imaginary po-edges before the
>> RMW read and before the RMW write, and extending the smp_mb() ordering
>> rule, which currently only applies to real po edges that would be found
>> around a really inserted smp_mb(), also to cases of the only imagined po
>> edges.
>>
>> Reported-by: Viktor Vafeiadis <viktor@...-sws.org>
>> Suggested-by: Alan Stern <stern@...land.harvard.edu>
>> Signed-off-by: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
>> ---
>>   tools/memory-model/linux-kernel.cat | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
>> index adf3c4f41229..d7e7bf13c831 100644
>> --- a/tools/memory-model/linux-kernel.cat
>> +++ b/tools/memory-model/linux-kernel.cat
>> @@ -34,6 +34,16 @@ let R4rmb = R \ Noreturn	(* Reads for which rmb works *)
>>   let rmb = [R4rmb] ; fencerel(Rmb) ; [R4rmb]
>>   let wmb = [W] ; fencerel(Wmb) ; [W]
>>   let mb = ([M] ; fencerel(Mb) ; [M]) |
>> +	(*
>> +	 * full-barrier RMWs (successful cmpxchg(), xchg(), etc.) act as
>> +	 * though there were enclosed by smp_mb().
>> +	 * The effect of these virtual smp_mb() is formalized by adding
>> +	 * Mb tags to the read and write of the operation, and providing
>> +	 * the same ordering as though there were additional po edges
>> +	 * between the Mb tag and the read resp. write.
>> +	 *)
>> +	([M] ; po ; [Mb & R]) |
>> +	([Mb & W] ; po ; [M]) |
> 
> I couldn't help suggestting:
> 
> 	([M] ; po ; [Mb & domain(rmw)]) |
> 	([Mb & range(rmw)] ; po ; [M]) |
> 
> , it's a bit more clear to me, but maybe the comment above is good
> enough?

Hm, maybe clarity is in the eye of the beholder in this case.

Actually looking at your suggestion makes me think of smp_store_mb(), 
which although represented as Once;F[Mb] could be (mis)understood also 
as Mb&W. And it indeed does the same thing
   ([Mb & W] ; po ; [M])
would suggest.

(btw I think it is confusing that smp_store_mb is not strictly stronger 
than smp_store_release. Of course there are places where you want a 
relaxed store followed by an mb, but usually the mb versions are 
strictly stronger.).

Best wishes,
   jonas