Messages in this thread | | | Date | Thu, 3 Aug 2017 17:12:24 +0100 | From | Will Deacon <> | Subject | Re: [RFC][PATCH v3]: documentation,atomic: Add new documents |
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On Thu, Aug 03, 2017 at 07:55:14AM -0700, Paul E. McKenney wrote: > On Thu, Aug 03, 2017 at 10:05:16PM +0800, Boqun Feng wrote: > > Hi Will, > > > > On Wed, Aug 02, 2017 at 10:45:32AM +0100, Will Deacon wrote: > > [...] > > > > > > It's worth noting that we don't have the problem with any value-returning > > > atomics, so all flavours of xchg in this test would be forbidden on arm64 > > > too. > > > > > > > C C-WillDeacon-MP+o-r+ai-rmb-o.litmus > > > > > > > > (* > > > > * Expected result: Never. > > > > * > > > > * Desired litmus test, with atomic_inc() emulated by xchg_relaxed(): > > > > * > > > > * WRITE_ONCE(x, 1); atomic_inc(&y); > > > > * r0 = xchg_release(&y, 5); smp_rmb(); > > > > * r1 = READ_ONCE(x); > > > > * > > > > * > > > > * WARN_ON(r0 == 0 && r1 == 0); > > > > *) > > > > > > > > { > > > > } > > > > > > > > P0(int *x, int *y) > > > > { > > > > WRITE_ONCE(*x, 1); > > > > r0 = xchg_release(y, 5); > > > > } > > > > > > > > P1(int *x, int *y) > > > > { > > > > r2 = xchg_relaxed(y, 1); > > > > smp_rmb(); > > > > r1 = READ_ONCE(*x); > > > > } > > > > > > > > exists > > > > (0:r0=0 /\ 1:r1=0) > > > > > > > > How about a litmus test like this? > > > > C C-AMO-global-transitivity.litmus > > > > { > > } > > > > P0(int *x, int *y) > > { > > WRITE_ONCE(*x, 1); > > r0 = xchg_release(y, 5); > > } > > > > P1(int *y, int *z) > > { > > atomic_inc(y); > > smp_mb(); > > I am going to guess that the smp_mb() enforces the needed ordering, > but Will will let me know. ;-)
Yup, that would be forbidden on arm64, and would also be forbidden if you used WRITE_ONCE instead of atomic_inc (remember that we recently became multi-copy atomic).
Will
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