Re: [PATCH v2 2/2] riscv/atomic: Strengthen implementations with fences

[Alan Stern]

On Fri, 9 Mar 2018, 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)

There's another aspect to this imposed by the LKMM, and I'm not sure
whether your patch addresses it.  You add a fence after the cmpxchg
operation but nothing before it.  So what would happen with the 
following litmus test (which the LKMM forbids)?

C SB-atomic_cmpxchg-mb

{}

P0(int *x, int *y)
{
	int r0;

	WRITE_ONCE(*x, 1);
	r0 = atomic_cmpxchg(y, 0, 0);
}

P1(int *x, int *y)
{
	int r1;

	WRITE_ONCE(*y, 1);
	smp_mb();
	r1 = READ_ONCE(*x);
}

exists (0:r0=0 /\ 1:r1=0)

This is yet another illustration showing that full fences are stronger 
than cominations of release + acquire.

Alan Stern