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SubjectRe: [PATCH v3 1/5] tools/memory-model: Add an exception for limitations on _unless() family
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@inria.fr>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> 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
>

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