linux-kernel - Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

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Date:   Wed, 25 Jan 2023 14:10:08 +0100
From:   Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To:     paulmck@...nel.org, Alan Stern <stern@...land.harvard.edu>
Cc:     Andrea Parri <parri.andrea@...il.com>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
        dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
        urezki <urezki@...il.com>,
        quic_neeraju <quic_neeraju@...cinc.com>,
        frederic <frederic@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)



On 1/25/2023 3:20 AM, Paul E. McKenney wrote:
> On Tue, Jan 24, 2023 at 08:54:56PM -0500, Alan Stern wrote:
>> On Tue, Jan 24, 2023 at 02:54:49PM -0800, Paul E. McKenney wrote:
>>> On Tue, Jan 24, 2023 at 05:35:33PM -0500, Alan Stern wrote:
>>>> Can you be more explicit?  Exactly what guarantees does the kernel
>>>> implementation make that can't be expressed in LKMM?
>>> I doubt that I will be able to articulate it very well, but here goes.
>>>
>>> Within the Linux kernel, the rule for a given RCU "domain" is that if
>>> an event follows a grace period in pretty much any sense of the word,
>>> then that event sees the effects of all events in all read-side critical
>>> sections that began prior to the start of that grace period.
>>>
>>> Here the senses of the word "follow" include combinations of rf, fr,
>>> and co, combined with the various acyclic and irreflexive relations
>>> defined in LKMM.
>> The LKMM says pretty much the same thing.  In fact, it says the event
>> sees the effects of all events po-before the unlock of (not just inside)
>> any read-side critical section that began prior to the start of the
>> grace period.
>>
>>>> And are these anything the memory model needs to worry about?
>>> Given that several people, yourself included, are starting to use LKMM
>>> to analyze the Linux-kernel RCU implementations, maybe it does.
>>>
>>> Me, I am happy either way.
>> Judging from your description, I don't think we have anything to worry
>> about.
> Sounds good, and let's proceed on that assumption then.  We can always
> revisit later if need be.
>
> 							Thanx, Paul

FWIW, I currently don't see a need for either RCU nor "base" LKMM to 
have this kind of guarantee.
But I'm curious for why it doesn't exist in LKMM -- is it because of 
Alpha or some other issues that make it hard to guarantee (like a 
compiler merging two threads and optimizing or something?), or is it 
simply that it seemed like a complicated guarantee with no discernible 
upside, or something else?

Best wishes, jonas