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Message-ID: <20200402080358.GC59159@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net>
Date: Thu, 2 Apr 2020 16:03:58 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Joel Fernandes <joel@...lfernandes.org>
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 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@...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>
Thanks!
Acked-by: Boqun Feng <boqun.feng@...il.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
>
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