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Date:   Thu, 2 Apr 2020 16:03:58 +0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     linux-kernel@vger.kernel.org,
        Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Rob Herring <robh@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        linux-arch@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH v4 0/4] Documentation/litmus-tests: Add litmus tests for
 atomic APIs
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On Wed, Apr 01, 2020 at 11:58:16PM -0400, Joel Fernandes wrote:
> 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>

Thanks!

Acked-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

> ---
>  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
>