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

Paul E. McKenney
2020-09-03T23:30:39Z

On Wed, Sep 02, 2020 at 08:37:15PM +0800, Boqun Feng wrote:
> On Wed, Sep 02, 2020 at 12:14:12PM +0200, peterz@infradead.org wrote:
> > 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.
> >
>
> IIUC, the DW colomn stands for whether the corresponding operation (in
> this case, it's atomic_set()) is ordered any write that depends on this
> operation. I don't think there is a write->write dependency, so DW for
> atomic_set() should not be Y, just as the DW for WRITE_ONCE().
>
> > > 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 ?
>
> Agreed. Probably we should fold those two operations into "Load" and
> "Store" cases.

All good points.

How about like this, adding "Relaxed" to the WRITE_ONCE() and READ_ONCE()
rows and "RMW" to the "Relaxed operation" row?

The file contents are followed by a diff against the previous version.

Thanx, Paul

------------------------------------------------------------------------

Prior Operation Subsequent Operation
--------------- ---------------------------
C Self R W RMW Self R W DR DW RMW SV
-- ---- - - --- ---- - - -- -- --- --

Relaxed store Y Y
Relaxed load Y Y Y Y
Relaxed RMW 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
smp_rmb() Y R Y Y R
smp_wmb() Y W Y Y W
smp_mb() & synchronize_rcu() CP Y Y Y Y Y Y Y Y
Successful full non-void RMW CP Y Y Y Y Y Y Y Y Y Y Y
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: Relaxed: A relaxed operation is either a *_relaxed() RMW
operation, an unsuccessful RMW operation, READ_ONCE(),
WRITE_ONCE(), or one of the atomic_read() and
atomic_set() family of operations.
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

------------------------------------------------------------------------

diff --git a/tools/memory-model/Documentation/cheatsheet.txt b/tools/memory-model/Documentation/cheatsheet.txt
index 31b814d..4146b8d 100644
--- a/tools/memory-model/Documentation/cheatsheet.txt
+++ b/tools/memory-model/Documentation/cheatsheet.txt
@@ -3,9 +3,9 @@
C Self R W RMW Self R W DR DW RMW SV
-- ---- - - --- ---- - - -- -- --- --

-Store, e.g., WRITE_ONCE() Y Y
-Load, e.g., READ_ONCE() Y Y Y Y
-Relaxed operation Y Y Y Y
+Relaxed store Y Y
+Relaxed load Y Y Y Y
+Relaxed RMW 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
@@ -18,8 +18,9 @@ smp_mb__after_atomic() CP a a Y Y Y Y Y Y

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.
+ operation, an unsuccessful RMW operation, READ_ONCE(),
+ WRITE_ONCE(), or one of the atomic_read() and
+ atomic_set() family of operations.
C: Ordering is cumulative
P: Ordering propagates
R: Read, for example, READ_ONCE(), or read portion of RMW