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

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Date:   Wed, 29 Nov 2017 14:18:48 -0800
From:   Daniel Lustig <dlustig@...dia.com>
To:     "paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
        Alan Stern <stern@...land.harvard.edu>
CC:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        Will Deacon <will.deacon@....com>,
        David Howells <dhowells@...hat.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Unlock-lock questions and the Linux Kernel Memory Model

On 11/29/2017 12:42 PM, Paul E. McKenney wrote:
> On Wed, Nov 29, 2017 at 02:53:06PM -0500, Alan Stern wrote:
>> On Wed, 29 Nov 2017, Peter Zijlstra wrote:
>>
>>> On Wed, Nov 29, 2017 at 11:04:53AM -0800, Daniel Lustig wrote:
>>>
>>>> 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.
>>>
>>> That's just weird. Either its _relaxed, or its _release. Making _relaxed
>>> mean _release is just daft.
>>
>> The C11 memory model specifically allows atomic operations to be 
>> interspersed within a release sequence.  But it doesn't say why.
> 
> The use case put forward within the committee is for atomic quantities
> with mode bits.  The most frequent has the atomic quantity having
> lock-like properties, in which case you don't want to lose the ordering
> effects of the lock handoff just because a mode bit got set or cleared.
> Some claim to actually use something like this, but details have not
> been forthcoming.
> 
> I confess to being a bit skeptical.  If the mode changes are infrequent,
> the update could just as well be ordered.

Aren't reference counting implementations which use memory_order_relaxed
for incrementing the count another important use case?  Specifically,
the synchronization between a memory_order_release decrement and the
eventual memory_order_acquire/consume free shouldn't be interrupted by
other (relaxed) increments and (release-only) decrements that happen in
between.  At least that's my understanding of this use case.  I wasn't
there when the C/C++ committee decided this.

> That said, Daniel, the C++ memory model really does require that the
> above litmus test be forbidden, my denigration of it notwithstanding.

Yes I agree, that's why I'm curious what the Linux memory model has
in mind here :)

Dan

> 							Thanx, Paul
>