linux-kernel - Re: [tip:locking/core] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

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Message-Id: <1631847849.o57vj41jx3.astroid@bobo.none>
Date:   Fri, 17 Sep 2021 13:21:52 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>,
        Will Deacon <will@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        dlustig@...dia.com, Stephane Eranian <eranian@...gle.com>,
        Peter Anvin <hpa@...or.com>, Jiri Olsa <jolsa@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-tip-commits@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        mpe@...erman.id.au, palmer@...belt.com,
        Andrea Parri <parri.andrea@...il.com>,
        paul.walmsley@...ive.com, Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Vince Weaver <vincent.weaver@...ne.edu>
Subject: Re: [tip:locking/core] tools/memory-model: Add extra ordering for
 locks and remove it for ordinary release/acquire

Excerpts from Will Deacon's message of September 10, 2021 9:08 pm:
> Hi Paul,
> 
> On Thu, Sep 09, 2021 at 10:46:35AM -0700, Paul E. McKenney wrote:
>> On Thu, Sep 09, 2021 at 02:35:36PM +0100, Will Deacon wrote:
>> > On Thu, Sep 09, 2021 at 09:25:30AM +0200, Peter Zijlstra wrote:
>> > > On Wed, Sep 08, 2021 at 09:08:33AM -0700, Linus Torvalds wrote:
>> > > > then I think it's entirely reasonable to
>> > > > 
>> > > >         spin_unlock(&r);
>> > > >         spin_lock(&s);
>> > > > 
>> > > > cannot be reordered.
>> > > 
>> > > I'm obviously completely in favour of that :-)
>> > 
>> > I don't think we should require the accesses to the actual lockwords to
>> > be ordered here, as it becomes pretty onerous for relaxed LL/SC
>> > architectures where you'd end up with an extra barrier either after the
>> > unlock() or before the lock() operation. However, I remain absolutely in
>> > favour of strengthening the ordering of the _critical sections_ guarded by
>> > the locks to be RCsc.
>> 
>> If by this you mean the critical sections when observed only by other
>> critical sections for a given lock, then everyone is already there.
> 
> No, I mean the case where somebody without the lock (but using memory
> barriers) can observe the critical sections out of order (i.e. W -> R
> order is not maintained).

This is a sincere question, why is this important? I mean _any_ 
restriction on reordering makes things easier by definition I can't 
argue with that, but why is this one in particular seen as a problem?
It just seems disproportionate.

We naturally think of accesses within locks as atomic as a whole 
(provided the other parties are doing the proper locking too). So like 
atomic operations, aligned stores, etc can be reordered, I don't see why
these should have any particular ordering either, or why a unlock() 
should pair with a later lock() of an unrelated lock to provide some
ordering.

It gives the idea that individual lock operations in isolation should be 
or do something special, but I think that's the wrong way to think about 
it, the lock and the unlock operate on a specific lock word and protect 
specific data vs other processors that access the same data under the
same locks.

If you don't know what you're doing or don't want to think about 
ordering, perform accesses under locks. If you don't lock, you get to
think about ordering. At which point sure two sets of operations from
different critical sections could go out of order, but so can any two
stores. Or two stores from inside the one critical section if you are
not holding the correct lock.

Thanks,
Nick