[PATCH V2 0/3] riscv: atomic: Optimize AMO instructions usage

[Guo Ren]

Hi Boqun & Andrea,

On Sun, Apr 17, 2022 at 10:26 AM Boqun Feng <boqun.feng at gmail.com> wrote:
>
> On Sun, Apr 17, 2022 at 12:49:44AM +0800, Guo Ren wrote:
> [...]
> >
> > If both the aq and rl bits are set, the atomic memory operation is
> > sequentially consistent and cannot be observed to happen before any
> > earlier memory operations or after any later memory operations in the
> > same RISC-V hart and to the same address domain.
> >                 "0:     lr.w     %[p],  %[c]\n"
> >                 "       sub      %[rc], %[p], %[o]\n"
> >                 "       bltz     %[rc], 1f\n".
> > -               "       sc.w.rl  %[rc], %[rc], %[c]\n"
> > +               "       sc.w.aqrl %[rc], %[rc], %[c]\n"
> >                 "       bnez     %[rc], 0b\n"
> > -               "       fence    rw, rw\n"
> >                 "1:\n"
> > So .rl + fence rw, rw is over constraints, only using sc.w.aqrl is more proper.
> >
>
> Can .aqrl order memory accesses before and after it (not against itself,
> against each other), i.e. act as a full memory barrier? For example, can
>From the RVWMO spec description, the .aqrl annotation appends the same
effect with "fence rw, rw" to the AMO instruction, so it's RCsc.

Not only .aqrl, and I think the below also could be an RCsc when
sc.w.aq is executed:
A: Pre-Access
B: lr.w.rl ADDR-0
...
C: sc.w.aq ADDR-0
D: Post-Acess
Because sc.w.aq has overlap address & data dependency on lr.w.rl, the
global memory order should be A->B->C->D when sc.w.aq is executed. For
the amoswap

The purpose of the whole patchset is to reduce the usage of
independent fence rw, rw instructions, and maximize the usage of the
.aq/.rl/.aqrl aonntation of RISC-V.

                __asm__ __volatile__ (                                  \
                        "0:     lr.w %0, %2\n"                          \
                        "       bne  %0, %z3, 1f\n"                     \
                        "       sc.w.rl %1, %z4, %2\n"                  \
                        "       bnez %1, 0b\n"                          \
                        "       fence rw, rw\n"                         \
                        "1:\n"                                          \

> we end up with u == 1, v == 1, r1 on P0 is 0 and r1 on P1 is 0, for the
> following litmus test?
>
>     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)
I think my patchset won't affect the above sequence guarantee. Current
RISC-V implementation only gives RCsc when the original value is the
same at least once. So I prefer RISC-V cmpxchg should be:


-                       "0:     lr.w %0, %2\n"                          \
+                      "0:     lr.w.rl %0, %2\n"                          \
                        "       bne  %0, %z3, 1f\n"                     \
                        "       sc.w.rl %1, %z4, %2\n"                  \
                        "       bnez %1, 0b\n"                          \
-                       "       fence rw, rw\n"                         \
                        "1:\n"                                          \
+                        "       fence w, rw\n"                    \

To give an unconditional RSsc for atomic_cmpxchg.

>
> Regards,
> Boqun



-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/