From: Alan Stern <stern@rowland.harvard.edu>
To: Andrea Parri <parri.andrea@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
"Reshetova, Elena" <elena.reshetova@intel.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
"keescook@chromium.org" <keescook@chromium.org>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"mingo@redhat.com" <mingo@redhat.com>,
"ishkamiel@gmail.com" <ishkamiel@gmail.com>,
Will Deacon <will.deacon@arm.com>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
<boqun.feng@gmail.com>, <dhowells@redhat.com>,
<david@fromorbit.com>
Subject: Re: [PATCH] refcount: provide same memory ordering guarantees as in atomic_t
Date: Thu, 2 Nov 2017 16:28:45 -0400 (EDT) [thread overview]
Message-ID: <Pine.LNX.4.44L0.1711021622121.1277-100000@iolanthe.rowland.org> (raw)
In-Reply-To: <20171102174504.GA19833@andrea>
On Thu, 2 Nov 2017, Andrea Parri wrote:
> > This is forbidden. It would remain forbidden even if the smp_mb in P1
> > were replaced by a similar release/acquire pair for the same memory
> > location.
>
> Hopefully, the LKMM does not agree with this assessment... ;-)
No, it doesn't.
> Here's a two-threads example showing that "(w)mb is _not_ rfi-rel-acq":
>
> C rfi-rel-acq-is-not-mb
>
> {}
>
> P0(int *x, int *y, int *a)
> {
> WRITE_ONCE(*x, 1);
> smp_store_release(a, 1);
> r1 = smp_load_acquire(a);
> WRITE_ONCE(*y, 1);
> }
>
> P1(int *x, int *y)
> {
> int r0;
> int r1;
>
> r0 = READ_ONCE(*y);
> smp_rmb();
> r1 = READ_ONCE(*x);
> }
>
> exists (1:r0=1 /\ 1:r1=0)
Right. There is a happens-before edge between the two WRITE_ONCE calls
in P0 but no cumul-fence edge, and therefore the test is allowed.
Here is an example where a happens-before edge suffices to provide
ordering:
P0(int *x, int *y)
{
WRITE_ONCE(*x, 1);
smp_wmb();
WRITE_ONCE(*y, 1);
}
P1(int *x, int *y, int *a)
{
int rx, ry, ra;
ry = READ_ONCE(*y);
smp_store_release(a, 1);
ra = smp_load_acquire(a);
rx = READ_ONCE(*x);
}
exists (1:rx=0 /\ 1:ry=1)
This test is forbidden, but it would be allowed if the release and
acquire accessed different locations.
Alan Stern
next prev parent reply other threads:[~2017-11-02 20:28 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-23 11:09 [PATCH] refcount: provide same memory ordering guarantees as in atomic_t Elena Reshetova
2017-10-23 13:12 ` Peter Zijlstra
2017-10-27 6:49 ` Reshetova, Elena
2017-10-27 13:56 ` Peter Zijlstra
2017-11-02 11:04 ` Reshetova, Elena
2017-11-02 13:57 ` Peter Zijlstra
2017-11-02 15:40 ` Alan Stern
2017-11-02 16:02 ` Peter Zijlstra
2017-11-02 16:45 ` Peter Zijlstra
2017-11-02 17:08 ` Alan Stern
2017-11-02 17:16 ` Will Deacon
2017-11-02 17:26 ` Peter Zijlstra
2017-11-02 20:21 ` Alan Stern
2017-11-15 18:05 ` Will Deacon
2017-11-15 19:15 ` Alan Stern
2017-11-15 20:03 ` Peter Zijlstra
2017-11-15 20:22 ` Alan Stern
2017-11-16 8:46 ` Peter Zijlstra
2017-11-15 21:01 ` Andrea Parri
2017-11-16 8:58 ` Peter Zijlstra
2017-11-16 10:00 ` Andrea Parri
2017-11-02 17:45 ` Andrea Parri
2017-11-02 20:28 ` Alan Stern [this message]
2017-11-03 11:55 ` Reshetova, Elena
2017-11-13 9:09 ` Reshetova, Elena
2017-11-13 13:19 ` Paul E. McKenney
2017-11-13 16:01 ` Reshetova, Elena
2017-11-13 16:26 ` Paul E. McKenney
2017-11-14 11:23 ` Reshetova, Elena
2017-11-14 17:24 ` Paul E. McKenney
2017-11-16 13:44 ` Michal Hocko
2017-11-16 15:29 ` Paul E. McKenney
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