linux-kernel - Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}

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Date:   Fri, 19 Apr 2019 20:00:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Will Deacon <will.deacon@....com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided
 by smp_mb__{before,after}_atomic()

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.