lkml.org 
[lkml]   [2017]   [Jul]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC][PATCH v3]: documentation,atomic: Add new documents
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
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2017-07-31 13:04    [W:0.480 / U:0.072 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site