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

Boqun Feng
2020-09-02T03:54:56Z

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 ;-)

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?

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
> --
> 2.9.5
>