From: Alan Stern <stern@rowland.harvard.edu>
To: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: 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>,
"Paul E. McKenney" <paulmck@linux.ibm.com>,
Peter Zijlstra <peterz@infradead.org>,
Will Deacon <will.deacon@arm.com>,
Dmitry Vyukov <dvyukov@google.com>,
Nick Desaulniers <ndesaulniers@google.com>,
<linux-kernel@vger.kernel.org>
Subject: Re: Plain accesses and data races in the Linux Kernel Memory Model
Date: Thu, 17 Jan 2019 14:43:54 -0500 (EST) [thread overview]
Message-ID: <Pine.LNX.4.44L0.1901171121360.1207-100000@iolanthe.rowland.org> (raw)
In-Reply-To: <20190116213658.GA3984@andrea>
On Wed, 16 Jan 2019, Andrea Parri wrote:
> Can the compiler (maybe, it does?) transform, at the C or at the "asm"
> level, LB1's P0 in LB2's P0 (LB1 and LB2 are reported below)?
>
> C LB1
>
> {
> int *x = &a;
> }
>
> P0(int **x, int *y)
> {
> int *r0;
>
> r0 = rcu_dereference(*x);
> *r0 = 0;
> smp_wmb();
> WRITE_ONCE(*y, 1);
> }
>
> P1(int **x, int *y, int *b)
> {
> int r0;
>
> r0 = READ_ONCE(*y);
> rcu_assign_pointer(*x, b);
> }
>
> exists (0:r0=b /\ 1:r0=1)
>
>
> C LB2
>
> {
> int *x = &a;
> }
>
> P0(int **x, int *y)
> {
> int *r0;
>
> r0 = rcu_dereference(*x);
> if (*r0)
> *r0 = 0;
> smp_wmb();
> WRITE_ONCE(*y, 1);
> }
>
> P1(int **x, int *y, int *b)
> {
> int r0;
>
> r0 = READ_ONCE(*y);
> rcu_assign_pointer(*x, b);
> }
>
> exists (0:r0=b /\ 1:r0=1)
>
> LB1 and LB2 are data-race free, according to the patch; LB1's "exists"
> clause is not satisfiable, while LB2's "exists" clause is satisfiable.
Umm. Transforming
*r0 = 0;
to
if (*r0 != 0)
*r0 = 0;
wouldn't work on Alpha if r0 was assigned from a plain read with no
memory barrier between. But when r0 is assigned from an
rcu_dereference call, or if there's no indirection (as in "if (a != 0)
a = 0;"), the compiler is indeed allowed to perform this
transformation.
This means my definition of preserved writes was wrong; a write we
thought had to be preserved could instead be transformed into a read.
This objection throws a serious monkey wrench into my approach. For
one thing, it implies that (as in the example) we can't expect
smp_wmb() always to order plain writes. For another, it means we have
to assume a lot more writes need not be preserved.
I don't know. This may doom the effort to formalize dependencies to
plain accesses. Or at least, those other than address dependencies
from marked reads.
Alan
next prev parent reply other threads:[~2019-01-17 19:43 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <Pine.LNX.4.44L0.1901141439480.1366-100000@iolanthe.rowland.org>
[not found] ` <20190114235426.GV1215@linux.ibm.com>
2019-01-15 7:20 ` Plain accesses and data races in the Linux Kernel Memory Model Dmitry Vyukov
2019-01-15 15:03 ` Alan Stern
2019-01-15 15:23 ` Paul E. McKenney
2019-01-15 14:25 ` Andrea Parri
2019-01-15 15:19 ` Alan Stern
2019-01-16 11:57 ` Peter Zijlstra
2019-01-16 13:11 ` Paul E. McKenney
2019-01-16 15:49 ` Alan Stern
2019-01-16 21:36 ` Andrea Parri
2019-01-17 15:03 ` Andrea Parri
2019-01-17 20:21 ` Alan Stern
2019-01-18 15:10 ` Alan Stern
2019-01-18 15:56 ` Andrea Parri
2019-01-18 16:43 ` Alan Stern
2019-01-17 19:43 ` Alan Stern [this message]
2019-01-18 18:53 ` Paul E. McKenney
2019-01-22 15:47 ` Andrea Parri
2019-01-22 16:19 ` Alan Stern
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