Re: [PATCH v4 0/4] Documentation/litmus-tests: Add litmus tests for atomic APIs

Joel Fernandes
2020-04-02T03:58:21Z

On Tue, Mar 31, 2020 at 09:40:37AM +0800, Boqun Feng wrote:
> On Fri, Mar 27, 2020 at 06:18:43PM -0400, Joel Fernandes wrote:
> > On Thu, Mar 26, 2020 at 10:40:18AM +0800, Boqun Feng wrote:
> > > A recent discussion raises up the requirement for having test cases for
> > > atomic APIs:
> > >
> > > https://lore.kernel.org/lkml/20200213085849.GL14897@hirez.programming.kicks-ass.net/
> > >
> > > , and since we already have a way to generate a test module from a
> > > litmus test with klitmus[1]. It makes sense that we add more litmus
> > > tests for atomic APIs. And based on the previous discussion, I create a
> > > new directory Documentation/atomic-tests and put these litmus tests
> > > here.
> > >
> > > This patchset starts the work by adding the litmus tests which are
> > > already used in atomic_t.txt, and also improve the atomic_t.txt to make
> > > it consistent with the litmus tests.
> > >
> > > Previous version:
> > > v1: https://lore.kernel.org/linux-doc/20200214040132.91934-1-boqun.feng@gmail.com/
> > > v2: https://lore.kernel.org/lkml/20200219062627.104736-1-boqun.feng@gmail.com/
> > > v3: https://lore.kernel.org/linux-doc/20200227004049.6853-1-boqun.feng@gmail.com/
> >
> > For full series:
> >
> > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> >
> > One question I had was in the existing atomic_set() documentation, it talks
> > about atomic_add_unless() implementation based on locking could have issues.
> > It says the way to fix such cases is:
> >
> > Quote:
> > the typical solution is to then implement atomic_set{}() with
> > atomic_xchg().
> >
> > I didn't get how using atomic_xchg() fixes it. Is the assumption there that
> > atomic_xchg() would be implemented using locking to avoid atomic_set() having
>
> Right, I think that's the intent of the sentence.
>
> > issues? If so, we could clarify that in the document.
> >
>
> Patches are welcome ;-)

---8<-----------------------

Like this? I'll add it to my tree and send it to Paul during my next
series, unless you disagree ;-)

Subject: [PATCH] doc: atomic_t: Document better about the locking within
atomic_xchg()

It is not fully clear how the atomic_set() would not cause an issue with
preservation of the atomicity of RMW in this example. Make it clear that
locking within atomic_xchg() would save the day.

Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
Documentation/atomic_t.txt | 2 ++
1 file changed, 2 insertions(+)

diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index 0f1fdedf36bbb..1d9c307c73a7c 100644
--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -129,6 +129,8 @@ with a lock:
unlock();

the typical solution is to then implement atomic_set{}() with atomic_xchg().
+The locking within the atomic_xchg() in CPU1 would ensure that the value read
+in CPU0 would not be overwritten.

RMW ops:
--
2.26.0.292.g33ef6b2f38-goog