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

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Message-ID: <20200402035816.GA46686@google.com>
Date:   Wed, 1 Apr 2020 23:58:16 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jonathan Corbet <corbet@....net>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Rob Herring <robh@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 0/4] Documentation/litmus-tests: Add litmus tests for
 atomic APIs

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@...lfernandes.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@...il.com>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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