linux-kernel - Re: Unlock-lock questions and the Linux Kernel Memory Model

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Date:   Wed, 29 Nov 2017 11:04:53 -0800
From:   Daniel Lustig <dlustig@...dia.com>
To:     Alan Stern <stern@...land.harvard.edu>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Jade Alglave <j.alglave@....ac.uk>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        David Howells <dhowells@...hat.com>,
        Palmer Dabbelt <palmer@...belt.com>
CC:     Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Unlock-lock questions and the Linux Kernel Memory Model

On 11/27/2017 1:16 PM, Alan Stern wrote:
> This is essentially a repeat of an email I sent out before the
> Thanksgiving holiday, the assumption being that lack of any responses
> was caused by the holiday break.  (And this time the message is CC'ed
> to LKML, so there will be a public record of it.)
> 
> A few people have said they believe the Linux Kernel Memory Model
> should make unlock followed by lock (of the same variable) act as a
> write memory barrier.  In other words, they want the memory model to
> forbid the following litmus test:
>
<snip>
> 
> I (and others!) would like to know people's opinions on these matters.
> 
> Alan Stern

While we're here, let me ask about another test which isn't directly
about unlock/lock but which is still somewhat related to this
discussion:

"MP+wmb+xchg-acq" (or some such)

{}

P0(int *x, int *y)
{
        WRITE_ONCE(*x, 1);
        smp_wmb();
        WRITE_ONCE(*y, 1);
}

P1(int *x, int *y)
{
        r1 = atomic_xchg_relaxed(y, 2);
        r2 = smp_load_acquire(y);
        r3 = READ_ONCE(*x);
}

exists (1:r1=1 /\ 1:r2=2 /\ 1:r3=0)

C/C++ would call the atomic_xchg_relaxed part of a release sequence
and hence would forbid this outcome.

x86 and Power would forbid this.  ARM forbids this via a special-case
rule in the memory model, ordering atomics with later load-acquires.

RISC-V, however, wouldn't forbid this by default using RCpc or RCsc
atomics for smp_load_acquire().  It's an "fri; rfi" type of pattern,
because xchg doesn't have an inherent internal data dependency.

If the Linux memory model is going to forbid this outcome, then
RISC-V would either need to use fences instead, or maybe we'd need to
add a special rule to our memory model similarly.  This is one detail
where RISC-V is still actively deciding what to do.

Have you all thought about this test before?  Any idea which way you
are leaning regarding the outcome above?

Thanks,
Dan