Messages in this thread | | | Date | Mon, 23 Jan 2023 21:18:14 -0500 | From | Alan Stern <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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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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