Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic()

From: Peter Zijlstra <peterz@infradead.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
	Andrea Parri <andrea.parri@amarulasolutions.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Daniel Lustig <dlustig@nvidia.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	Nicholas Piggin <npiggin@gmail.com>,
	"Paul E. McKenney" <paulmck@linux.ibm.com>,
	Will Deacon <will.deacon@arm.com>,
	Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic()
Date: Fri, 19 Apr 2019 20:00:17 +0200	[thread overview]
Message-ID: <20190419180017.GP4038@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1904191312200.1406-100000@iolanthe.rowland.org>

On Fri, Apr 19, 2019 at 01:21:45PM -0400, Alan Stern wrote:
> Index: usb-devel/Documentation/atomic_t.txt
> ===================================================================
> --- usb-devel.orig/Documentation/atomic_t.txt
> +++ usb-devel/Documentation/atomic_t.txt
> @@ -171,7 +171,10 @@ The barriers:
>    smp_mb__{before,after}_atomic()
>  
>  only apply to the RMW ops and can be used to augment/upgrade the ordering
> -inherent to the used atomic op. These barriers provide a full smp_mb().
> +inherent to the used atomic op. Unlike normal smp_mb() barriers, they order
> +only the RMW op itself against the instructions preceding the
> +smp_mb__before_atomic() or following the smp_mb__after_atomic(); they do
> +not order instructions on the other side of the RMW op at all.

Now it is I who is confused; what?

	x = 1;
	smp_mb__before_atomic();
	atomic_add(1, &a);
	y = 1;

the stores to both x and y will be ordered as if an smp_mb() where
there. There is no order between a and y otoh.