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Message-ID: <Y88/5ib7zYl67mcE@rowland.harvard.edu>
Date: Mon, 23 Jan 2023 21:18:14 -0500
From: Alan Stern <stern@...land.harvard.edu>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Andrea Parri <parri.andrea@...il.com>,
Jonas Oberhauser <jonas.oberhauser@...wei.com>,
Peter Zijlstra <peterz@...radead.org>, will <will@...nel.org>,
"boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
dhowells <dhowells@...hat.com>,
"j.alglave" <j.alglave@....ac.uk>,
"luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
urezki <urezki@...il.com>,
quic_neeraju <quic_neeraju@...cinc.com>,
frederic <frederic@...nel.org>,
Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
test)
On Mon, Jan 23, 2023 at 12:16:59PM -0800, Paul E. McKenney wrote:
> One twist is that the design of both SRCU and RCU are stronger than LKMM
> requires, as illustrated by the litmus test at the end of this email.
>
> I believe that your proof outline above also covers this case, but I
> figure that I should ask.
This test is full of typos, and I guess that one of them seriously
affects the meaning, because as far as I can tell the corrected test is
allowed.
> C C-srcu-observed-2
>
> (*
> * Result: Sometimes
> *
> * But please note that the Linux-kernel SRCU implementation is designed
> * to provide Never.
> *)
>
> {}
>
> P0(int *x, int *y, int *z, struct srcu_struct *s)
> {
> int r1;
> int r2;
r2 is never used.
>
> r1 = srcu_read_lock(s);
> WRITE_ONCE(*y, 1);
> WRITE_ONCE(*x, 1);
> srcu_read_unlock(s, r3);
There is no r3; this should be r1.
> }
>
> P1(int *x, int *y, int *z, struct srcu_struct *s)
> {
> int r1;
> int r2;
r2 is never used.
>
> r1 = READ_ONCE(*y);
> synchronize_srcu(s);
> WRITE_ONCE(*z, 1);
> }
>
> P2(int *x, int *y, int *z, struct srcu_struct *s)
> {
> int r1;
r1 is never used; it should be r2.
>
> WRITE_ONCE(*z, 2);
> smp_mb();
> r2 = READ_ONCE(*x);
> }
>
> exists (1:r1=1 /\ 1:r2=0 /\ z=1)
1:r2 is never used. Apparently this should 2:r2.
Given those changes, the test can run as follows: P2 runs to completion,
writing z=2 and reading x=0. Then P0 runs to completion, writing y=1
and x=1. Then P1 runs to completion, reading y=1 and overwriting z=1.
Alan
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