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

[Boqun Feng <boqun.feng@gmail.com>]

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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.

> 
> 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?

Regards,
Boqun

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