Re: Adding plain accesses and detecting data races in the LKMM

[Paul E. McKenney]

On Sat, Apr 13, 2019 at 11:39:38PM +0200, Andrea Parri wrote:
> On Tue, Apr 09, 2019 at 08:01:32AM -0700, Paul E. McKenney wrote:
> > On Tue, Apr 09, 2019 at 03:36:18AM +0200, Andrea Parri wrote:
> > > > > The formula was more along the line of "do not assume either of these
> > > > > cases to hold; use barrier() is you need an unconditional barrier..."
> > > > > AFAICT, all current implementations of smp_mb__{before,after}_atomic()
> > > > > provides a compiler barrier with either barrier() or "memory" clobber.
> > > > 
> > > > Well, we have two reasonable choices: Say that 
> > > > smp_mb__{before,after}_atomic will always provide a compiler barrier, 
> > > > or don't say this.  I see no point in saying that the combination of 
> > > > Before-atomic followed by RMW provides a barrier.
> > > 
> > > ;-/ I'm fine with the first choice. I don't see how the second choice
> > > (this proposal/patch) would be consistent with some documentation and
> > > with the current implementations; for example,
> > > 
> > > 1) Documentation/atomic_t.txt says:
> > > 
> > > Thus:
> > > 
> > >   atomic_fetch_add();
> > > 
> > > is equivalent to:
> > > 
> > >   smp_mb__before_atomic();
> > >   atomic_fetch_add_relaxed();
> > >   smp_mb__after_atomic();
> > > 
> > > [...]
> > > 
> > > 2) Some implementations of the _relaxed() variants do not provide any
> > > compiler barrier currently.
> > 
> > But don't all implementations of smp_mb__before_atomic() and
> > smp_mb__after_atomic() currently supply a compiler barrier?
> 
> Yes, AFAICS, all implementations of smp_mb__{before,after}_atomic() currently
> supply a compiler barrier.
> 
> Nevertheless, there's a difference between:  (1) Specify that these barriers
> supply a compiler barrier,  (2) Specify that (certain) combinations of these
> barriers and RMWs supply a compiler barrier, and  (3) This patch...  ;-)
> 
> FWIW, I'm not aware of current/informal documentation following (the arguably
> simpler but slightly stronger) (1).  But again (amending my last remark): (1)
> and (2) both make sense to me.

Another question is "should the kernel permit smp_mb__{before,after}*()
anywhere other than immediately before or after the primitive being
strengthened?"

Thoughts?

							Thanx, Paul