Re: [PATCH kcsan 6/9] tools/memory-model: Expand the cheatsheet.txt notion of relaxed

peterz
2020-09-02T10:14:28Z

On Wed, Sep 02, 2020 at 11:54:48AM +0800, Boqun Feng wrote:
> On Mon, Aug 31, 2020 at 11:20:34AM -0700, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> >
> > This commit adds a key entry enumerating the various types of relaxed
> > operations.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> > tools/memory-model/Documentation/cheatsheet.txt | 27 ++++++++++++++-----------
> > 1 file changed, 15 insertions(+), 12 deletions(-)
> >
> > diff --git a/tools/memory-model/Documentation/cheatsheet.txt b/tools/memory-model/Documentation/cheatsheet.txt
> > index 33ba98d..31b814d 100644
> > --- a/tools/memory-model/Documentation/cheatsheet.txt
> > +++ b/tools/memory-model/Documentation/cheatsheet.txt
> > @@ -5,7 +5,7 @@
> >
> > Store, e.g., WRITE_ONCE() Y Y
> > Load, e.g., READ_ONCE() Y Y Y Y
> > -Unsuccessful RMW operation Y Y Y Y
> > +Relaxed operation Y Y Y Y
> > rcu_dereference() Y Y Y Y
> > Successful *_acquire() R Y Y Y Y Y Y
> > Successful *_release() C Y Y Y W Y
> > @@ -17,14 +17,17 @@ smp_mb__before_atomic() CP Y Y Y a a a a Y
> > smp_mb__after_atomic() CP a a Y Y Y Y Y Y
> >
> >
> > -Key: C: Ordering is cumulative
> > - P: Ordering propagates
> > - R: Read, for example, READ_ONCE(), or read portion of RMW
> > - W: Write, for example, WRITE_ONCE(), or write portion of RMW
> > - Y: Provides ordering
> > - a: Provides ordering given intervening RMW atomic operation
> > - DR: Dependent read (address dependency)
> > - DW: Dependent write (address, data, or control dependency)
> > - RMW: Atomic read-modify-write operation
> > - SELF: Orders self, as opposed to accesses before and/or after
> > - SV: Orders later accesses to the same variable
> > +Key: Relaxed: A relaxed operation is either a *_relaxed() RMW
> > + operation, an unsuccessful RMW operation, or one of
> > + the atomic_read() and atomic_set() family of operations.
>
> To be accurate, atomic_set() doesn't return any value, so it cannot be
> ordered against DR and DW ;-)

Surely DW is valid for any store.

> I think we can split the Relaxed family into two groups:
>
> void Relaxed: atomic_set() or atomic RMW operations that don't return
> any value (e.g atomic_inc())
>
> non-void Relaxed: a *_relaxed() RMW operation, an unsuccessful RMW
> operation, or atomic_read().
>
> And "void Relaxed" is similar to WRITE_ONCE(), only has "Self" and "SV"
> equal "Y", while "non-void Relaxed" plays the same rule as "Relaxed"
> in this patch.
>
> Thoughts?

I get confused by the mention of all this atomic_read() atomic_set()
crud in the first place, why are they called out specifically from any
other regular load/store ?