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

Boqun Feng
2020-09-04T00:59:37Z

On Thu, Sep 03, 2020 at 04:30:37PM -0700, Paul E. McKenney wrote:
> 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?
>

Much better now, thanks! However ...

> 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

void Relaxed RMW operation is still missing ;-) Maybe:

void Relaxed RMW operation 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.

And:
a RMW operation that doesn't return any value (e.g
atomic_inc()), IOW it's a void Relaxed operation.

Thoughts?

Regards,
Boqun

> 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