Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Sat, 27 Apr 2019 01:36:29 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Andrea Parri <andrea.parri@amarulasolutions.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Kernel development list <linux-kernel@vger.kernel.org>,
        Luc Maranget <luc.maranget@inria.fr>,
        Alan Stern <stern@rowland.harvard.edu>,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided
 by smp_mb__{before,after}_atomic()
Reply-To: paulmck@linux.ibm.com
References: <Pine.LNX.4.44L0.1904191312200.1406-100000@iolanthe.rowland.org>
 <20190419180017.GP4038@hirez.programming.kicks-ass.net>
 <20190419182620.GF14111@linux.ibm.com>
 <1555719429.t9n8gkf70y.astroid@bobo.none>
 <20190420085440.GK14111@linux.ibm.com>
 <20190423123209.GR4038@hirez.programming.kicks-ass.net>
 <20190423133010.GK3923@linux.ibm.com>
 <20190427081738.GA11149@andrea>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190427081738.GA11149@andrea>
User-Agent: Mutt/1.5.21 (2010-09-15)
Message-Id: <20190427083629.GJ3923@linux.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sat, Apr 27, 2019 at 10:17:38AM +0200, Andrea Parri wrote:
> On Tue, Apr 23, 2019 at 06:30:10AM -0700, Paul E. McKenney wrote:
> > On Tue, Apr 23, 2019 at 02:32:09PM +0200, Peter Zijlstra wrote:
> > > On Sat, Apr 20, 2019 at 01:54:40AM -0700, Paul E. McKenney wrote:
> > > > 	And atomic_set(): set_preempt_state().	This fails
> > > > 	on x86, s390, and TSO friends, does it not?  Or is
> > > > 	this ARM-only?	Still, why not just smp_mb() before and
> > > > 	after?	Same issue in __kernfs_new_node(), bio_cnt_set(),
> > > > 	sbitmap_queue_update_wake_batch(), 
> > > > 
> > > > 	Ditto for atomic64_set() in __ceph_dir_set_complete().
> > > > 
> > > > 	Ditto for atomic_read() in rvt_qp_is_avail().  This function
> > > > 	has a couple of other oddly placed smp_mb__before_atomic().
> > > 
> > > That are just straight up bugs. The atomic_t.txt file clearly specifies
> > > the barriers only apply to RmW ops and both _set() and _read() are
> > > specified to not be a RmW.
> > 
> > Agreed.  The "Ditto" covers my atomic_set() consternation.  ;-)
> 
> I was working on some of these before the Easter break [1, 2]: the plan
> was to continue next week, but by addressing the remaining cases with a
> conservative s/that barrier/smp_mb at first; unless you've other plans?
> 
>   Andrea
> 
> [1] http://lkml.kernel.org/r/1555417031-27356-1-git-send-email-andrea.parri@amarulasolutions.com
> [2] http://lkml.kernel.org/r/1555404968-39927-1-git-send-email-pbonzini@redhat.com

Sounds good to me!  ;-)

								Thanx, Paul