Re: Adding plain accesses and detecting data races in the LKMM
From: Andrea Parri
Date: Mon Apr 08 2019 - 21:36:33 EST
> > 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.
Andrea