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

From: Guo Ren
Date: Sun Apr 24 2022 - 03:52:26 EST

On Fri, Apr 22, 2022 at 11:11 AM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> On Fri, Apr 22, 2022 at 09:56:21AM +0800, Guo Ren wrote:
> > On Fri, Apr 22, 2022 at 6:56 AM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
> > >
> > > On Thu, Apr 21, 2022 at 05:39:09PM +0800, Guo Ren wrote:
> > > > Hi Dan,
> > > >
> > > > On Thu, Apr 21, 2022 at 1:03 AM Dan Lustig <dlustig@xxxxxxxxxx> wrote:
> > > > >
> > > > > On 4/20/2022 1:33 AM, Guo Ren wrote:
> > > > > > Thx Dan,
> > > > > >
> > > > > > On Wed, Apr 20, 2022 at 1:12 AM Dan Lustig <dlustig@xxxxxxxxxx> wrote:
> > > > > >>
> > > > > >> On 4/17/2022 12:51 AM, Guo Ren wrote:
> > > > > >>> Hi Boqun & Andrea,
> > > > > >>>
> > > > > >>> On Sun, Apr 17, 2022 at 10:26 AM Boqun Feng <boqun.feng@xxxxxxxxx> 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
> > > > > >>
> > > > > >> These opcodes aren't actually meaningful, unfortunately.
> > > > > >>
> > > > > >> Quoting the ISA manual chapter 10.2: "Software should not set the rl bit
> > > > > >> on an LR instruction unless the aq bit is also set, nor should software
> > > > > >> set the aq bit on an SC instruction unless the rl bit is also set."
> > > > > > 1. Oh, I've missed the behind half of the ISA manual. But why can't we
> > > > > > utilize lr.rl & sc.aq in software programming to guarantee the
> > > > > > sequence?
> > > > >
> > > > > lr.aq and sc.rl map more naturally to hardware than lr.rl and sc.aq.
> > > > > Plus, they just aren't common operations to begin with, e.g., there
> > > > > is no smp_store_acquire() or smp_load_release(), nor are there
> > > > > equivalents in C/C++ atomics.
> > > > First, thx for pointing out that my patch violates the rules defined
> > > > in the ISA manual. I've abandoned these parts in v3.
> > > >
> > > > It's easy to let hw support lr.rl & sc.aq (eg: our hardware supports
> > > > them). I agree there are no equivalents in C/C++ atomics. But they are
> > > > useful for LR/SC pairs to implement atomic_acqurie/release semantics.
> > > > Compare below:
> > > > A): fence rw, r; lr
> > > > B): lr.rl
> > > > The A has another "fence ,r" effect in semantics, it's over commit
> > > > from a software design view.
> > > >
> > > > ps: Current definition has problems:
> > > > #define RISCV_ACQUIRE_BARRIER "\tfence r , rw\n"
> > > > #define RISCV_RELEASE_BARRIER "\tfence rw, w\n"
> > > >
> > > > #define __cmpxchg_release(ptr, old, new, size) \
> > > > ...
> > > > __asm__ __volatile__ ( \
> > > > RISCV_RELEASE_BARRIER \
> > > > "0: lr.w %0, %2\n" \
> > > >
> > > > That means "fence rw, w" can't prevent lr.w beyond the fence, we need
> > > > a "fence.rw. r" here. Here is the Fixup patch which I'm preparing:
> > > >
> > >
> > > That's not true. Note that RELEASE semantics only applies to the
> > > write/store part of a read-modify-write atomic, similarly, ACQUIRE only
> > I just want to point out that the "atomic" mentioned here is only for
> > RISC-V LR/SC AMO instructions. It has been clarified to tread AMO
> > instruction as the whole part for other AMO instructions.
> >
> > - .aq: If the aq bit is set, then no later memory operations
> > in this RISC-V hart can be observed to take place
> > before the AMO.
> > - .rl: If the rl bit is set, then other RISC-V harts will not
> > observe the AMO before memory accesses preceding the
> > AMO in this RISC-V hart.
> > - .aqrl: Setting both the aq and the rl bit on an AMO makes the
> > sequence sequentially consistent, meaning that it cannot
> > be reordered with earlier or later memory operations
> > from the same hart.
> >
> > > applies to the read/load part. For example, the following litmus test
> > > can observe the exists clause being true.
> > Thx for pointing out, that means changing "fence rw, w" to "fence rw.
> > r" is more strict and it would lower performance, right?
>
> Yes, I think it's more strict but honestly I don't know the performance
> impact ;-)
>
> >
> > >
> > > {}
> > >
> > > P0(int *x, int *y)
> > > {
> > > int r0;
> > > int r1;
> > >
> > > r0 = cmpxchg_acquire(x, 0, 1);
> > > r1 = READ_ONCE(*y);
> > Oh, READ_ONCE could be beyond the write/store part of cmpxchg_acquire,
> > right? We shouldn't prevent it.
>
> Right, the reordering is allowed by the API of Linux atomics and you
> don't have to prevent it.
Thx, you are right, I got it.

>
> Regards,
> Boqun
>
> >
> > > }
> > >
> > > P1(int *x, int *y)
> > > {
> > > int r0;
> > >
> > > WRITE_ONCE(*y, 1);
> > > smp_mb();
> > > r0 = READ_ONCE(*x);
> > > }
> > >
> > > exists (0:r0=0 /\ 0:r1=0 /\ 1:r0=0)
> > >
> > > Regards,
> > > Boqun
> > >
> > > > From 14c93aca0c3b10cf134791cf491b459972a36ec4 Mon Sep 17 00:00:00 2001
> > > > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > > > Date: Thu, 21 Apr 2022 16:44:48 +0800
> > > > Subject: [PATCH] riscv: atomic: Fixup wrong __atomic_acquire/release_fence
> > > > implementation
> > > >
> > > > Current RISCV_ACQUIRE/RELEASE_BARRIER is for spin_lock not atomic.
> > > >
> > > > __cmpxchg_release(ptr, old, new, size)
> > > > ...
> > > > __asm__ __volatile__ (
> > > > RISCV_RELEASE_BARRIER
> > > > "0: lr.w %0, %2\n"
> > > >
> > > > The "fence rw, w -> lr.w" is invalid and lr would beyond fence, so
> > > > we need "fence rw, r -> lr.w" here. Atomic acquire is the same.
> > > >
> > > > Fixes: 0123f4d76ca6 ("riscv/spinlock: Strengthen implementations with fences")
> > > > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > > > Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
> > > > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > > > Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
> > > > Cc: Dan Lustig <dlustig@xxxxxxxxxx>
> > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > ---
> > > > arch/riscv/include/asm/atomic.h | 4 ++--
> > > > arch/riscv/include/asm/cmpxchg.h | 8 ++++----
> > > > arch/riscv/include/asm/fence.h | 4 ++++
> > > > 3 files changed, 10 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/arch/riscv/include/asm/atomic.h b/arch/riscv/include/asm/atomic.h
> > > > index aef8aa9ac4f4..7cd66eba6ec3 100644
> > > > --- a/arch/riscv/include/asm/atomic.h
> > > > +++ b/arch/riscv/include/asm/atomic.h
> > > > @@ -20,10 +20,10 @@
> > > > #include <asm/barrier.h>
> > > >
> > > > #define __atomic_acquire_fence() \
> > > > - __asm__ __volatile__(RISCV_ACQUIRE_BARRIER "" ::: "memory")
> > > > + __asm__ __volatile__(RISCV_ATOMIC_ACQUIRE_BARRIER "":::"memory")
> > > >
> > > > #define __atomic_release_fence() \
> > > > - __asm__ __volatile__(RISCV_RELEASE_BARRIER "" ::: "memory");
> > > > + __asm__ __volatile__(RISCV_ATOMIC_RELEASE_BARRIER"" ::: "memory");
> > > >
> > > > static __always_inline int arch_atomic_read(const atomic_t *v)
> > > > {
> > > > diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> > > > index 9269fceb86e0..605edc2fca3b 100644
> > > > --- a/arch/riscv/include/asm/cmpxchg.h
> > > > +++ b/arch/riscv/include/asm/cmpxchg.h
> > > > @@ -217,7 +217,7 @@
> > > > " bne %0, %z3, 1f\n" \
> > > > " sc.w %1, %z4, %2\n" \
> > > > " bnez %1, 0b\n" \
> > > > - RISCV_ACQUIRE_BARRIER \
> > > > + RISCV_ATOMIC_ACQUIRE_BARRIER \
> > > > "1:\n" \
> > > > : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> > > > : "rJ" ((long)__old), "rJ" (__new) \
> > > > @@ -229,7 +229,7 @@
> > > > " bne %0, %z3, 1f\n" \
> > > > " sc.d %1, %z4, %2\n" \
> > > > " bnez %1, 0b\n" \
> > > > - RISCV_ACQUIRE_BARRIER \
> > > > + RISCV_ATOMIC_ACQUIRE_BARRIER \
> > > > "1:\n" \
> > > > : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> > > > : "rJ" (__old), "rJ" (__new) \
> > > > @@ -259,7 +259,7 @@
> > > > switch (size) { \
> > > > case 4: \
> > > > __asm__ __volatile__ ( \
> > > > - RISCV_RELEASE_BARRIER \
> > > > + RISCV_ATOMIC_RELEASE_BARRIER \
> > > > "0: lr.w %0, %2\n" \
> > > > " bne %0, %z3, 1f\n" \
> > > > " sc.w %1, %z4, %2\n" \
> > > > @@ -271,7 +271,7 @@
> > > > break; \
> > > > case 8: \
> > > > __asm__ __volatile__ ( \
> > > > - RISCV_RELEASE_BARRIER \
> > > > + RISCV_ATOMIC_RELEASE_BARRIER \
> > > > "0: lr.d %0, %2\n" \
> > > > " bne %0, %z3, 1f\n" \
> > > > " sc.d %1, %z4, %2\n" \
> > > > diff --git a/arch/riscv/include/asm/fence.h b/arch/riscv/include/asm/fence.h
> > > > index 2b443a3a487f..4e446d64f04f 100644
> > > > --- a/arch/riscv/include/asm/fence.h
> > > > +++ b/arch/riscv/include/asm/fence.h
> > > > @@ -4,9 +4,13 @@
> > > > #ifdef CONFIG_SMP
> > > > #define RISCV_ACQUIRE_BARRIER "\tfence r , rw\n"
> > > > #define RISCV_RELEASE_BARRIER "\tfence rw, w\n"
> > > > +#define RISCV_ATOMIC_ACQUIRE_BARRIER "\tfence w , rw\n"
> > > > +#define RISCV_ATOMIC_RELEASE_BARRIER "\tfence rw, r\n"
> > > > #else
> > > > #define RISCV_ACQUIRE_BARRIER
> > > > #define RISCV_RELEASE_BARRIER
> > > > +#define RISCV_ATOMIC_ACQUIRE_BARRIER
> > > > +#define RISCV_ATOMIC_RELEASE_BARRIER
> > > > #endif
> > > >
> > > > #endif /* _ASM_RISCV_FENCE_H */
> > > >
> > > >
> > > > >
> > > > > > 2. Using .aqrl to replace the fence rw, rw is okay to ISA manual,
> > > > > > right? And reducing a fence instruction to gain better performance:
> > > > > > "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"
> > > > >
> > > > > Yes, using .aqrl is valid.
> > > > Thx and I think the below is also valid, right?
> > > >
> > > > - RISCV_RELEASE_BARRIER \
> > > > - " amoswap.w %0, %2, %1\n" \
> > > > + " amoswap.w.rl %0, %2, %1\n" \
> > > >
> > > > - " amoswap.d %0, %2, %1\n" \
> > > > - RISCV_ACQUIRE_BARRIER \
> > > > + " amoswap.d.aq %0, %2, %1\n" \
> > > >
> > > > >
> > > > > Dan
> > > > >
> > > > > >>
> > > > > >> Dan
> > > > > >>
> > > > > >>> 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/
> >
> >
> >
> > --
> > Best Regards
> > Guo Ren
> >
> > ML: https://lore.kernel.org/linux-csky/

--
Best Regards
Guo Ren

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