linux-kernel - Re: [PATCH] tools/memory-model: remove rb-dep, smp_read_barrier_depends, and lockless

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Date:   Mon, 19 Feb 2018 11:41:23 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Andrea Parri <parri.andrea@...il.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        mingo@...nel.org, Will Deacon <will.deacon@....com>,
        peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH] tools/memory-model: remove rb-dep,
 smp_read_barrier_depends, and lockless_dereference

On Mon, Feb 19, 2018 at 12:14:45PM -0500, Alan Stern wrote:
> On Sat, 17 Feb 2018, Andrea Parri wrote:
> 
> > > Akira's observation about READ_ONCE extends to all (annotated) loads.  In
> > > fact, it also applies to loads corresponding to unsuccessful RMW operations;
> > > consider, for example, the following variation of MP+onceassign+derefonce:
> > > 
> > > C T
> > > 
> > > {
> > > y=z;
> > > z=0;
> > > }
> > > 
> > > P0(int *x, int **y)
> > > {
> > > 	WRITE_ONCE(*x, 1);
> > > 	smp_store_release(y, x);
> > > }
> > > 
> > > P1(int **y, int *z)
> > > {
> > > 	int *r0;
> > > 	int r1;
> > > 
> > > 	r0 = cmpxchg_relaxed(y, z, z);
> > > 	r1 = READ_ONCE(*r0);
> > > }
> > > 
> > > exists (1:r0=x /\ 1:r1=0)
> > > 
> > > The final state is allowed w/o the patch, and forbidden w/ the patch.
> > > 
> > > This also reminds me of
> > > 
> > >    5a8897cc7631fa544d079c443800f4420d1b173f
> > >    ("locking/atomics/alpha: Add smp_read_barrier_depends() to _release()/_relaxed() atomics")
> > > 
> > > (that we probably want to mention in the commit message).
> > 
> > Please also notice that 5a8897cc7631f only touched alpha's atomic.h:
> > I see no corresponding commit/change on {,cmp}xchg.h (where the "mb"
> > is currently conditionally executed).
> 
> This leaves us with a question: Do we want to change the kernel by
> adding memory barriers after unsuccessful RMW operations on Alpha, or
> do we want to change the model by excluding such operations from
> address dependencies?

I vote for adding the barrier on Alpha.  However, I don't know of any
code in the Linux kernel that relies on read-to-read address dependency
ordering headed by a failing RMW operation, so I don't feel all that
strongly about this.

> Note that operations like atomic_add_unless() already include memory 
> barriers.

And I don't see an atomic_add_unless_relaxed(), so we are good on this
one.  So far, anyway!  ;-)

							Thanx, Paul