From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: Akira Yokosawa <akiyks@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>,
Alan Stern <stern@rowland.harvard.edu>,
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>,
Peter Zijlstra <peterz@infradead.org>,
Will Deacon <will.deacon@arm.com>,
Daniel Kroening <kroening@cs.ox.ac.uk>,
Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Adding plain accesses and detecting data races in the LKMM
Date: Fri, 19 Apr 2019 18:37:37 +0200 [thread overview]
Message-ID: <20190419163737.GA8690@andrea> (raw)
In-Reply-To: <2827195a-f203-b9cd-444d-cf6425cef06f@gmail.com>
> > + (1) The compiler can reorder the load from a to precede the
> > + atomic_dec(), (2) Because x86 smp_mb__before_atomic() is only a
> > + compiler barrier, the CPU can reorder the preceding store to
> > + obj->dead with the later load from a.
> > +
> > + This could be avoided by using READ_ONCE(), which would prevent the
> > + compiler from reordering due to both atomic_dec() and READ_ONCE()
> > + being volatile accesses, and is usually preferable for loads from
> > + shared variables. However, weakly ordered CPUs would still be
> > + free to reorder the atomic_dec() with the load from a, so a more
> > + readable option is to also use smp_mb__after_atomic() as follows:
> > +
> > + WRITE_ONCE(obj->dead, 1);
> > + smp_mb__before_atomic();
> > + atomic_dec(&obj->ref_count);
> > + smp_mb__after_atomic();
> > + r1 = READ_ONCE(a);
>
> The point here is not just "readability", but also the portability of the
> code, isn't it?
The implicit assumption was, I guess, that all weakly ordered CPUs which
are free to reorder the atomic_dec() with the READ_ONCE() execute a full
memory barrier in smp_mb__before_atomic() ... This assumption currently
holds, AFAICT, but yes: it may well become "non-portable"! ... ;-)
Andrea
next prev parent reply other threads:[~2019-04-19 19:22 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-19 19:38 Adding plain accesses and detecting data races in the LKMM Alan Stern
2019-04-02 14:42 ` Andrea Parri
2019-04-02 18:06 ` Alan Stern
2019-04-06 0:49 ` Andrea Parri
2019-04-06 16:03 ` Alan Stern
2019-04-08 5:51 ` Andrea Parri
2019-04-08 14:18 ` Alan Stern
2019-04-09 1:36 ` Andrea Parri
2019-04-09 15:01 ` Paul E. McKenney
2019-04-13 21:39 ` Andrea Parri
2019-04-15 13:35 ` Paul E. McKenney
2019-04-15 13:50 ` Alan Stern
2019-04-15 13:53 ` Paul E. McKenney
2019-04-18 12:54 ` Andrea Parri
2019-04-18 17:44 ` Alan Stern
2019-04-18 18:39 ` Paul E. McKenney
2019-04-18 20:19 ` Alan Stern
2019-04-19 0:53 ` Andrea Parri
2019-04-19 12:47 ` Paul E. McKenney
2019-04-19 14:34 ` Alan Stern
2019-04-19 17:17 ` Paul E. McKenney
2019-04-19 15:06 ` Akira Yokosawa
2019-04-19 16:37 ` Andrea Parri [this message]
2019-04-19 18:06 ` Paul E. McKenney
2019-04-20 14:50 ` Akira Yokosawa
2019-04-21 19:38 ` Paul E. McKenney
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