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Date:   Wed, 18 Jan 2023 20:57:22 +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>
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From:   Jonas Oberhauser <s9joober@gmail.com>
In-Reply-To: <20230118035041.GQ2948950@paulmck-ThinkPad-P17-Gen-1>
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On 1/18/2023 4:50 AM, Paul E. McKenney wrote:
> On Tue, Jan 17, 2023 at 03:15:06PM -0500, Alan Stern wrote:
>> On Tue, Jan 17, 2023 at 09:43:08AM -0800, Paul E. McKenney wrote:
>>> On Tue, Jan 17, 2023 at 10:56:34AM -0500, Alan Stern wrote:
>>>> Isn't it true that the current code will flag srcu-bad-nesting if a
>>>> litmus test has non-nested overlapping SRCU read-side critical sections?
>>> Now that you mention it, it does indeed, flagging srcu-bad-nesting.
>>>
>>> Just to see if I understand, different-values yields true if the set
>>> contains multiple elements with the same value mapping to different
>>> values.  Or, to put it another way, if the relation does not correspond
>>> to a function.
>> As I understand it, given a relation r (i.e., a set of pairs of events),
>> different-values(r) returns the sub-relation consisting of those pairs
>> in r for which the value associated with the first event of the pair is
>> different from the value associated with the second event of the pair.
> OK, so different-values(r) is different than (r \ id) because the
> former operates on values and the latter on events?

I think you can say that (if you allow yourself to be a little bit loose 
with words, as I allow myself to be, much to the chagrin of Alan :) :( :)).

If you had a .value functional relation that relates every event to the 
value of that event, then
    different-values(r) = r \ .value ; .value^-1
i.e., it relates events x and y iff: 1) r relates x and y, and 2) the 
value of x is not equal to the value of y.

You could write this as
    different-values(r) = r \ .value ; value-id ; .value^-1
where value-id is like id but for values, i.e., relates every value v to 
itself.

You could say that this difference operates on the values of the events, 
rather than on the events itself.
In contrast,
     r \ id
works directly on the events and relates x and y iff: 1) r relates x and 
y, and 2) the event x is not equal to the event y.

In this sense I think your characterization is appropriate.