Re: [PATCH v3 1/5] tools/memory-model: Add an exception for limitations on _unless() family

[Alan Stern]

On Thu, 27 Feb 2020, Boqun Feng wrote:

> According to Luc, atomic_add_unless() is directly provided by herd7,
> therefore it can be used in litmus tests. So change the limitation
> section in README to unlimit the use of atomic_add_unless().

Is this really true?  Why does herd treat atomic_add_unless() different
from all the other atomic RMS ops?  All the other ones we support do
have entries in linux-kernel.def.

Alan

PS: It seems strange to support atomic_add_unless but not 
atomic_long_add_unless.  The difference between the two is trivial.

> 
> Cc: Luc Maranget <luc.maranget@xxxxxxxx>
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> ---
>  tools/memory-model/README | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/memory-model/README b/tools/memory-model/README
> index fc07b52f2028..409211b1c544 100644
> --- a/tools/memory-model/README
> +++ b/tools/memory-model/README
> @@ -207,11 +207,15 @@ The Linux-kernel memory model (LKMM) has the following limitations:
>  		case as a store release.
>  
>  	b.	The "unless" RMW operations are not currently modeled:
> -		atomic_long_add_unless(), atomic_add_unless(),
> -		atomic_inc_unless_negative(), and
> -		atomic_dec_unless_positive().  These can be emulated
> +		atomic_long_add_unless(), atomic_inc_unless_negative(),
> +		and atomic_dec_unless_positive().  These can be emulated
>  		in litmus tests, for example, by using atomic_cmpxchg().
>  
> +		One exception of this limitation is atomic_add_unless(),
> +		which is provided directly by herd7 (so no corresponding
> +		definition in linux-kernel.def). atomic_add_unless() is
> +		modeled by herd7 therefore it can be used in litmus tests.
> +
>  	c.	The call_rcu() function is not modeled.  It can be
>  		emulated in litmus tests by adding another process that
>  		invokes synchronize_rcu() and the body of the callback
>