linux-kernel - Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

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Message-Id: <20170731174345.GL3730@linux.vnet.ibm.com>
Date:   Mon, 31 Jul 2017 10:43:45 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

On Mon, Jul 31, 2017 at 07:04:03PM +0800, Boqun Feng wrote:
> On Mon, Jul 31, 2017 at 11:05:35AM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 26, 2017 at 08:47:50PM +0800, Boqun Feng wrote:
> > 
> > > > +
> > > > +Further, while something like:
> > > > +
> > > > +  smp_mb__before_atomic();
> > > > +  atomic_dec(&X);
> > > > +
> > > > +is a 'typical' RELEASE pattern, the barrier is strictly stronger than
> > > > +a RELEASE. Similarly for something like:
> > > > +
> > > 
> > > .. at here. Maybe you planned to put stronger ACQUIRE pattern?
> > 
> > Yes, although I struggled to find a sensible one. The problem is that
> > ACQUIRE is on loads and value returning atomics have an ACQUIRE variant,
> > so why would you ever want to use smp_mb__after_atomic() for this.
> > 
> > 
> > That is, the best I could come up with is something like:
> > 
> > 	val = atomic_fetch_or_relaxed(1, &var);
> > 	smp_mb__after_atomic();
> > 
> > But in that case we should've just written:
> > 
> > 	val = atomic_fetch_or_acquire(1, &var);
> > 
> 
> Agreed.
> 
> And besides, in memory-barriers.txt, the wording is:
> 
>  (*) smp_mb__before_atomic();
>  (*) smp_mb__after_atomic();
> 
>      These are for use with atomic (such as add, subtract, increment and
>      decrement) functions that don't return a value, especially when used for
>      reference counting. 
> 
> So actually, using smp_mb__after_atomic() for ACQUIRE is a misuse.

You lost me on this one.

Why wouldn't the following have ACQUIRE semantics?

	atomic_inc(&var);
	smp_mb__after_atomic();

Is the issue that there is no actual value returned or some such?

> > Suggestions?
> 
> As a result, I think it's better we say smp_mb__{before,after}_atomic()
> are only for 1) non-value-returning RmW atomic ops, 2)
> {set,clear,change}_bit and 3) internal use of atomic primitives(e.g. the
> generic version of fully ordered atomics).
> 
> 1) prevents people to use it for an ACQUIRE, but allows for a RELEASE.
> 1) & 2) makes atomic_t.txt consistent with memory-barriers.txt
> 3) explains our usage of those barriers internally.
> 
> Thoughts?

So if I have something like this, the assertion really can trigger?

	WRITE_ONCE(x, 1);		atomic_inc(&y);
	r0 = xchg_release(&y, 5);	smp_mb__after_atomic();
					r1 = READ_ONCE(x);


	WARN_ON(r0 == 0 && r1 == 0);

I must confess that I am not seeing why we would want to allow this
outcome.

							Thanx, Paul