Message-ID: <cedf3a39-12cd-1cb1-ad5a-7c10768cee40@huaweicloud.com>
Date:   Wed, 25 Jan 2023 14:10:08 +0100
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Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)
To:     paulmck@kernel.org, Alan Stern <stern@rowland.harvard.edu>
Cc:     Andrea Parri <parri.andrea@gmail.com>,
        Jonas Oberhauser <jonas.oberhauser@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>, will <will@kernel.org>,
        "boqun.feng" <boqun.feng@gmail.com>, npiggin <npiggin@gmail.com>,
        dhowells <dhowells@redhat.com>,
        "j.alglave" <j.alglave@ucl.ac.uk>,
        "luc.maranget" <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
        dlustig <dlustig@nvidia.com>, joel <joel@joelfernandes.org>,
        urezki <urezki@gmail.com>,
        quic_neeraju <quic_neeraju@quicinc.com>,
        frederic <frederic@kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
References: <20230124145423.GI2948950@paulmck-ThinkPad-P17-Gen-1>
 <8cc799ab-ffa1-47f7-6e1d-97488a210f14@huaweicloud.com>
 <20230124162253.GL2948950@paulmck-ThinkPad-P17-Gen-1>
 <3e5020c2-0dd3-68a6-9b98-5a7f57ed7733@huaweicloud.com>
 <20230124172647.GN2948950@paulmck-ThinkPad-P17-Gen-1>
 <2788294a-972e-acbc-84ce-25d2bb4d26d6@huaweicloud.com>
 <20230124221524.GV2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y9BdNVk2LQiUYABS@rowland.harvard.edu>
 <20230124225449.GY2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y9CL8LBz+/mbbD00@rowland.harvard.edu>
 <20230125022019.GB2948950@paulmck-ThinkPad-P17-Gen-1>
From:   Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
In-Reply-To: <20230125022019.GB2948950@paulmck-ThinkPad-P17-Gen-1>
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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