On Mon, Mar 05, 2018 at 07:24:36PM +0100, Andrea Parri wrote:
> Atomics present the same issue with locking: release and acquire
> variants need to be strengthened to meet the constraints defined
> by the Linux-kernel memory consistency model [1].
>
> Atomics present a further issue: implementations of atomics such
> as atomic_cmpxchg() and atomic_add_unless() rely on LR/SC pairs,
> which do not give full-ordering with .aqrl; for example, current
> implementations allow the "lr-sc-aqrl-pair-vs-full-barrier" test
> below to end up with the state indicated in the "exists" clause.
>
> In order to "synchronize" LKMM and RISC-V's implementation, this
> commit strengthens the implementations of the atomics operations
> by replacing .rl and .aq with the use of ("lightweigth") fences,
> and by replacing .aqrl LR/SC pairs in sequences such as:
>
> 0: lr.w.aqrl %0, %addr
> bne %0, %old, 1f
> ...
> sc.w.aqrl %1, %new, %addr
> bnez %1, 0b
> 1:
>
> with sequences of the form:
>
> 0: lr.w %0, %addr
> bne %0, %old, 1f
> ...
> sc.w.rl %1, %new, %addr /* SC-release */
> bnez %1, 0b
> fence rw, rw /* "full" fence */
> 1:
>
> following Daniel's suggestion.
>
> These modifications were validated with simulation of the RISC-V
> memory consistency model.
>
> C lr-sc-aqrl-pair-vs-full-barrier
>
> {}
>
> P0(int *x, int *y, atomic_t *u)
> {
> int r0;
> int r1;
>
> WRITE_ONCE(*x, 1);
> r0 = atomic_cmpxchg(u, 0, 1);
> r1 = READ_ONCE(*y);
> }
>
> P1(int *x, int *y, atomic_t *v)
> {
> int r0;
> int r1;
>
> WRITE_ONCE(*y, 1);
> r0 = atomic_cmpxchg(v, 0, 1);
> r1 = READ_ONCE(*x);
> }
>
> exists (u=1 /\ v=1 /\ 0:r1=0 /\ 1:r1=0)
>
> [1] https://marc.info/?l=linux-kernel&m=151930201102853&w=2
>
> https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/hKywNHBkAXM
> https://marc.info/?l=linux-kernel&m=151633436614259&w=2
>
> Suggested-by: Daniel Lustig <[email protected]>
> Signed-off-by: Andrea Parri <[email protected]>
> Cc: Palmer Dabbelt <[email protected]>
> Cc: Albert Ou <[email protected]>
> Cc: Daniel Lustig <[email protected]>
> Cc: Alan Stern <[email protected]>
> Cc: Will Deacon <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Boqun Feng <[email protected]>
> Cc: Nicholas Piggin <[email protected]>
> Cc: David Howells <[email protected]>
> Cc: Jade Alglave <[email protected]>
> Cc: Luc Maranget <[email protected]>
> Cc: "Paul E. McKenney" <[email protected]>
> Cc: Akira Yokosawa <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Linus Torvalds <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
[...]
> +static __always_inline
> \
> +c_t atomic##prefix##_xchg_release(atomic##prefix##_t *v, c_t n)
> \
> +{ \
> + return __xchg_acquire(&(v->counter), n, size); \
> +} \
[...]
> +static __always_inline
> \
> +c_t atomic##prefix##_cmpxchg_release(atomic##prefix##_t *v, \
> + c_t o, c_t n) \
> +{ \
> + return __cmpxchg_acquire(&(v->counter), o, n, size); \
> +} \
These better be _release in v2 ...
Andrea
