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: Fri, 18 Jan 2019 11:43:02 -0500 (EST) [thread overview]
Message-ID: <Pine.LNX.4.44L0.1901181137180.1425-100000@iolanthe.rowland.org> (raw)
In-Reply-To: <20190118155638.GA24442@andrea>
On Fri, 18 Jan 2019, Andrea Parri wrote:
> > A relatively simple solution to this problem would be to say that
> > smp_wmb doesn't order plain writes.
>
> It seems so; I don't have other solutions to suggest ATM. (But, TBH,
> I'm still in the process of reviewing/testing these changes... )
>
> And yes, this is a pain! : I don't have the exact statistics, but I'm
> willing to believe that removing this order will take us back ~99% of
> the current (~500!) uses of smp_wmb() ;-/
>
> Oh, well, maybe we'll find a better solution one day: after all, that
> one doesn't seem worse than what the current LKMM has to say! ;-)
>
>
> >
> > I think the rest of the memory model would then be okay. However, I am
> > open to arguments that this approach is too complex and we should
> > insist on the same kind of strict ordering for race avoidance that the
> > C11 standard uses (i.e., conflicting accesses separated by full memory
> > barriers or release & acquire barriers or locking).
>
> Indeed; maybe, we've just found another reason to obsolete smp_wmb()! ;-)
Here's another example of how smp_wmb can cause trouble. In this test,
I have replaced "*x = 1" in P1 with "r2 = *x; if (r2 != 1) *x = 1",
which is a perfectly valid transformation for the compiler to make.
But as a result of this transformation, the MP pattern between P1 and
P2 is now allowed!
This shows that when plain accesses are involved, smp_wmb() in the
writing thread is not sufficient to forbid MP.
Alan
C bad-wmb
{}
P0(int *x, int *y)
{
WRITE_ONCE(*x, 1);
smp_store_release(y, 1);
}
P1(int *x, int *y, int *z)
{
int r1;
int r2;
r1 = smp_load_acquire(y);
if (r1) {
/* Instead of *x = 1 ... */
r2 = *x;
if (r2 != 1)
*x = 1;
smp_wmb();
WRITE_ONCE(*z, 1);
}
}
P2(int *x, int *z)
{
int r3;
int r4 = 0;
r3 = READ_ONCE(*z);
if (r3) {
smp_rmb();
r4 = READ_ONCE(*x);
}
}
exists (2:r3=1 /\ 2:r4=0)
next prev parent reply other threads:[~2019-01-18 16:43 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 [this message]
2019-01-17 19:43 ` Alan Stern
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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