Re: [PATCH v2] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic()

[Paul E. McKenney]

On Mon, May 06, 2019 at 06:42:38PM +0200, Andrea Parri wrote:
> On Fri, May 03, 2019 at 01:13:44PM -0400, Alan Stern wrote:
> > The description of smp_mb__before_atomic() and smp_mb__after_atomic()
> > in Documentation/atomic_t.txt is slightly terse and misleading.  It
> > does not clearly state which other instructions are ordered by these
> > barriers.
> > 
> > This improves the text to make the actual ordering implications clear,
> > and also to explain how these barriers differ from a RELEASE or
> > ACQUIRE ordering.
> > 
> > Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 
> I understand that this does indeed better describe the intended semantics:
> 
> Acked-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>

I reverted the original and applied this one.  It will become visible
at the next rebase.

> Now we would only need to fix the implementations and a few (mis)uses. ;-)

You do have a start on this task!  ;-)

							Thanx, Paul

>   Andrea
> 
> 
> > 
> > ---
> > 
> > v2: Update the explanation: These barriers do provide order for 
> > accesses on the far side of the atomic RMW operation.
> > 
> > 
> >  Documentation/atomic_t.txt |   17 +++++++++++++----
> >  1 file changed, 13 insertions(+), 4 deletions(-)
> > 
> > Index: usb-devel/Documentation/atomic_t.txt
> > ===================================================================
> > --- usb-devel.orig/Documentation/atomic_t.txt
> > +++ usb-devel/Documentation/atomic_t.txt
> > @@ -170,8 +170,14 @@ The barriers:
> >  
> >    smp_mb__{before,after}_atomic()
> >  
> > -only apply to the RMW ops and can be used to augment/upgrade the ordering
> > -inherent to the used atomic op. These barriers provide a full smp_mb().
> > +only apply to the RMW atomic ops and can be used to augment/upgrade the
> > +ordering inherent to the op. These barriers act almost like a full smp_mb():
> > +smp_mb__before_atomic() orders all earlier accesses against the RMW op
> > +itself and all accesses following it, and smp_mb__after_atomic() orders all
> > +later accesses against the RMW op and all accesses preceding it. However,
> > +accesses between the smp_mb__{before,after}_atomic() and the RMW op are not
> > +ordered, so it is advisable to place the barrier right next to the RMW atomic
> > +op whenever possible.
> >  
> >  These helper barriers exist because architectures have varying implicit
> >  ordering on their SMP atomic primitives. For example our TSO architectures
> > @@ -195,7 +201,9 @@ Further, while something like:
> >    atomic_dec(&X);
> >  
> >  is a 'typical' RELEASE pattern, the barrier is strictly stronger than
> > -a RELEASE. Similarly for something like:
> > +a RELEASE because it orders preceding instructions against both the read
> > +and write parts of the atomic_dec(), and against all following instructions
> > +as well. Similarly, something like:
> >  
> >    atomic_inc(&X);
> >    smp_mb__after_atomic();
> > @@ -227,7 +235,8 @@ strictly stronger than ACQUIRE. As illus
> >  
> >  This should not happen; but a hypothetical atomic_inc_acquire() --
> >  (void)atomic_fetch_inc_acquire() for instance -- would allow the outcome,
> > -since then:
> > +because it would not order the W part of the RMW against the following
> > +WRITE_ONCE.  Thus:
> >  
> >    P1			P2
> >  
> > 
>