From: Alan Stern <stern@rowland.harvard.edu>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Andrea Parri <andrea.parri@amarulasolutions.com>,
LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.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>,
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: Thu, 18 Apr 2019 16:19:29 -0400 (EDT) [thread overview]
Message-ID: <Pine.LNX.4.44L0.1904181608400.1303-100000@iolanthe.rowland.org> (raw)
In-Reply-To: <20190418183919.GO14111@linux.ibm.com>
On Thu, 18 Apr 2019, Paul E. McKenney wrote:
> > Are you saying that on x86, atomic_inc() acts as a full memory barrier
> > but not as a compiler barrier, and vice versa for
> > smp_mb__after_atomic()? Or that neither atomic_inc() nor
> > smp_mb__after_atomic() implements a full memory barrier?
> >
> > Either one seems like a very dangerous situation indeed.
>
> If I am following the macro-name breadcrumb trails correctly, x86's
> atomic_inc() does have a compiler barrier. But this is an accident
> of implementation -- from what I can see, it is not required to do so.
>
> So smb_mb__after_atomic() is only guaranteed to order the atomic_inc()
> before B, not A. To order A before B in the above example, an
> smp_mb__before_atomic() is also needed.
Are you certain?
> But now that I look, LKMM looks to be stating a stronger guarantee:
>
> ([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) |
> ([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) |
> ([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
> ([M] ; po ; [UL] ; (co | po) ; [LKW] ;
> fencerel(After-unlock-lock) ; [M])
>
> Maybe something like this?
>
> ([M] ; fencerel(Before-atomic) ; [RMW] ; fencerel(After-atomic) ; [M]) |
> ([M] ; fencerel(Before-atomic) ; [RMW] |
> ( [RMW] ; fencerel(After-atomic) ; [M]) |
> ([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
> ([M] ; po ; [UL] ; (co | po) ; [LKW] ;
> fencerel(After-unlock-lock) ; [M])
The first line you wrote is redundant; it follows from the second and
third lines.
Aside from that, while this proposal may be correct and may express
what smb_mb__{before|after}_atomic really are intended to do, it
contradicts Documentation/atomic_t.txt. That file says:
These barriers provide a full smp_mb().
And of course, a full smp_mb() would order everything before it against
everything after it. If we update the model then we should also update
that file.
In addition, it's noteworthy that smp_mb__after_spinlock and
smp_mb__after_unlock_lock do not behave in this way.
Alan
next prev parent reply other threads:[~2019-04-18 20:19 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 [this message]
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
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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