Re: [PATCH V4 5/5] riscv: atomic: Optimize LRSC-pairs atomic ops with .aqrl annotation

From: Palmer Dabbelt
Date: Thu Jun 02 2022 - 01:59:34 EST

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On Sun, 22 May 2022 06:12:56 PDT (-0700), guoren@xxxxxxxxxx wrote:

On Sun, May 22, 2022 at 4:46 AM Palmer Dabbelt <palmer@xxxxxxxxxxx> wrote:

On Wed, 04 May 2022 20:55:26 PDT (-0700), guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
>
> The current implementation is the same with 8e86f0b409a4
> ("arm64: atomics: fix use of acquire + release for full barrier
> semantics"). RISC-V could combine acquire and release into the SC
> instructions and it could reduce a fence instruction to gain better
> performance. Here is related descriptio from RISC-V ISA 10.2
> Load-Reserved/Store-Conditional Instructions:
>
> - .aq: The LR/SC sequence can be given acquire semantics by
> setting the aq bit on the LR instruction.
> - .rl: The LR/SC sequence can be given release semantics by
> setting the rl bit on the SC instruction.
> - .aqrl: Setting the aq bit on the LR instruction, and setting
> both the aq and the rl bit on the SC instruction makes
> the LR/SC sequence sequentially consistent, meaning that
> it cannot be reordered with earlier or later memory
> operations from the same hart.
>
> 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. LR.rl and SC.aq
> instructions are not guaranteed to provide any stronger ordering
> than those with both bits clear, but may result in lower
> performance.
>
> The only difference is when sc.w/d.aqrl failed, it would cause .aq
> effect than before. But it's okay for sematic because overlap
> address LR couldn't beyond relating SC.

IIUC that's not accurate, or at least wasn't in 2018. The ISA tends to
drift around a bit, so it's possible things have changed since then.
5ce6c1f3535f ("riscv/atomic: Strengthen implementations with fences")

Thx for explaining, that why I suspected with the sentence in the comment:
"which do not give full-ordering with .aqrl"

Sorry, I'm not quite sure what you're saying here. My understanding is that this change results in mappings that violate LKMM, based on the rationale in that previous commit. IIUC that all still holds, but I'm not really a memory model person so I frequently miss stuff around there.

describes the issue more specifically, that's when we added these
fences. There have certainly been complains that these fences are too
heavyweight for the HW to go fast, but IIUC it's the best option we have

Yeah, it would reduce the performance on D1 and our next-generation
processor has optimized fence performance a lot.

Definately a bummer that the fences make the HW go slow, but I don't really see any other way to go about this. If you think these mappings are valid for LKMM and RVWMO then we should figure this out, but trying to drop fences to make HW go faster in ways that violate the memory model is going to lead to insanity.

I can definately buy the argument that we have mappings of various application memory models that are difficult to make fast given the ISA encodings of RVWMO we have, but that's not really an issue we can fix in the atomic mappings.

given the current set of memory model primitives we implement in the
ISA (ie, there's more in RVWMO but no way to encode that).

The others all look good, though, and as these are really all
independent cleanups I'm going to go ahead and put those three on
for-next.

There's also a bunch of checkpatch errors. The ones about "*" seem
spurious, but the alignment ones aren't. Here's my fixups:

Thx for the fixup.

diff --git a/arch/riscv/include/asm/atomic.h b/arch/riscv/include/asm/atomic.h
index 34f757dfc8f2..0bde499fa8bc 100644
--- a/arch/riscv/include/asm/atomic.h
+++ b/arch/riscv/include/asm/atomic.h
@@ -86,9 +86,9 @@ ATOMIC_OPS(xor, xor, i)
* versions, while the logical ops only have fetch versions.
*/
#define ATOMIC_FETCH_OP(op, asm_op, I, asm_type, c_type, prefix) \
-static __always_inline \
-c_type arch_atomic##prefix##_fetch_##op##_relaxed(c_type i, \
- atomic##prefix##_t *v) \
+static __always_inline c_type \
+arch_atomic##prefix##_fetch_##op##_relaxed(c_type i, \
+ atomic##prefix##_t *v) \
{ \
register c_type ret; \
__asm__ __volatile__ ( \
@@ -98,9 +98,9 @@ c_type arch_atomic##prefix##_fetch_##op##_relaxed(c_type i, \
: "memory"); \
return ret; \
} \
-static __always_inline \
-c_type arch_atomic##prefix##_fetch_##op##_acquire(c_type i, \
- atomic##prefix##_t *v) \
+static __always_inline c_type \
+arch_atomic##prefix##_fetch_##op##_acquire(c_type i, \
+ atomic##prefix##_t *v) \
{ \
register c_type ret; \
__asm__ __volatile__ ( \
@@ -110,9 +110,9 @@ c_type arch_atomic##prefix##_fetch_##op##_acquire(c_type i, \
: "memory"); \
return ret; \
} \
-static __always_inline \
-c_type arch_atomic##prefix##_fetch_##op##_release(c_type i, \
- atomic##prefix##_t *v) \
+static __always_inline c_type \
+arch_atomic##prefix##_fetch_##op##_release(c_type i, \
+ atomic##prefix##_t *v) \
{ \
register c_type ret; \
__asm__ __volatile__ ( \
@@ -122,8 +122,8 @@ c_type arch_atomic##prefix##_fetch_##op##_release(c_type i, \
: "memory"); \
return ret; \
} \
-static __always_inline \
-c_type arch_atomic##prefix##_fetch_##op(c_type i, atomic##prefix##_t *v) \
+static __always_inline c_type \
+arch_atomic##prefix##_fetch_##op(c_type i, atomic##prefix##_t *v) \
{ \
register c_type ret; \
__asm__ __volatile__ ( \
@@ -135,28 +135,28 @@ c_type arch_atomic##prefix##_fetch_##op(c_type i, atomic##prefix##_t *v) \
}

#define ATOMIC_OP_RETURN(op, asm_op, c_op, I, asm_type, c_type, prefix) \
-static __always_inline \
-c_type arch_atomic##prefix##_##op##_return_relaxed(c_type i, \
- atomic##prefix##_t *v) \
+static __always_inline c_type \
+arch_atomic##prefix##_##op##_return_relaxed(c_type i, \
+ atomic##prefix##_t *v) \
{ \
- return arch_atomic##prefix##_fetch_##op##_relaxed(i, v) c_op I; \
+ return arch_atomic##prefix##_fetch_##op##_relaxed(i, v) c_op I; \
} \
-static __always_inline \
-c_type arch_atomic##prefix##_##op##_return_acquire(c_type i, \
- atomic##prefix##_t *v) \
+static __always_inline c_type \
+arch_atomic##prefix##_##op##_return_acquire(c_type i, \
+ atomic##prefix##_t *v) \
{ \
- return arch_atomic##prefix##_fetch_##op##_acquire(i, v) c_op I; \
+ return arch_atomic##prefix##_fetch_##op##_acquire(i, v) c_op I; \
} \
-static __always_inline \
-c_type arch_atomic##prefix##_##op##_return_release(c_type i, \
- atomic##prefix##_t *v) \
+static __always_inline c_type \
+arch_atomic##prefix##_##op##_return_release(c_type i, \
+ atomic##prefix##_t *v) \
{ \
- return arch_atomic##prefix##_fetch_##op##_release(i, v) c_op I; \
+ return arch_atomic##prefix##_fetch_##op##_release(i, v) c_op I; \
} \
-static __always_inline \
-c_type arch_atomic##prefix##_##op##_return(c_type i, atomic##prefix##_t *v) \
+static __always_inline c_type \
+arch_atomic##prefix##_##op##_return(c_type i, atomic##prefix##_t *v) \
{ \
- return arch_atomic##prefix##_fetch_##op(i, v) c_op I; \
+ return arch_atomic##prefix##_fetch_##op(i, v) c_op I; \
}

#ifdef CONFIG_GENERIC_ATOMIC64

>
> 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: Dan Lustig <dlustig@xxxxxxxxxx>
> Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
> ---
> arch/riscv/include/asm/atomic.h | 24 ++++++++----------------
> arch/riscv/include/asm/cmpxchg.h | 6 ++----
> 2 files changed, 10 insertions(+), 20 deletions(-)
>
> diff --git a/arch/riscv/include/asm/atomic.h b/arch/riscv/include/asm/atomic.h
> index 34f757dfc8f2..aef8aa9ac4f4 100644
> --- a/arch/riscv/include/asm/atomic.h
> +++ b/arch/riscv/include/asm/atomic.h
> @@ -269,9 +269,8 @@ static __always_inline int arch_atomic_fetch_add_unless(atomic_t *v, int a, int
> "0: lr.w %[p], %[c]\n"
> " beq %[p], %[u], 1f\n"
> " add %[rc], %[p], %[a]\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"
> : [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
> : [a]"r" (a), [u]"r" (u)
> @@ -290,9 +289,8 @@ static __always_inline s64 arch_atomic64_fetch_add_unless(atomic64_t *v, s64 a,
> "0: lr.d %[p], %[c]\n"
> " beq %[p], %[u], 1f\n"
> " add %[rc], %[p], %[a]\n"
> - " sc.d.rl %[rc], %[rc], %[c]\n"
> + " sc.d.aqrl %[rc], %[rc], %[c]\n"
> " bnez %[rc], 0b\n"
> - " fence rw, rw\n"
> "1:\n"
> : [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
> : [a]"r" (a), [u]"r" (u)
> @@ -382,9 +380,8 @@ static __always_inline bool arch_atomic_inc_unless_negative(atomic_t *v)
> "0: lr.w %[p], %[c]\n"
> " bltz %[p], 1f\n"
> " addi %[rc], %[p], 1\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"
> : [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
> :
> @@ -402,9 +399,8 @@ static __always_inline bool arch_atomic_dec_unless_positive(atomic_t *v)
> "0: lr.w %[p], %[c]\n"
> " bgtz %[p], 1f\n"
> " addi %[rc], %[p], -1\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"
> : [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
> :
> @@ -422,9 +418,8 @@ static __always_inline int arch_atomic_dec_if_positive(atomic_t *v)
> "0: lr.w %[p], %[c]\n"
> " addi %[rc], %[p], -1\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"
> : [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
> :
> @@ -444,9 +439,8 @@ static __always_inline bool arch_atomic64_inc_unless_negative(atomic64_t *v)
> "0: lr.d %[p], %[c]\n"
> " bltz %[p], 1f\n"
> " addi %[rc], %[p], 1\n"
> - " sc.d.rl %[rc], %[rc], %[c]\n"
> + " sc.d.aqrl %[rc], %[rc], %[c]\n"
> " bnez %[rc], 0b\n"
> - " fence rw, rw\n"
> "1:\n"
> : [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
> :
> @@ -465,9 +459,8 @@ static __always_inline bool arch_atomic64_dec_unless_positive(atomic64_t *v)
> "0: lr.d %[p], %[c]\n"
> " bgtz %[p], 1f\n"
> " addi %[rc], %[p], -1\n"
> - " sc.d.rl %[rc], %[rc], %[c]\n"
> + " sc.d.aqrl %[rc], %[rc], %[c]\n"
> " bnez %[rc], 0b\n"
> - " fence rw, rw\n"
> "1:\n"
> : [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
> :
> @@ -486,9 +479,8 @@ static __always_inline s64 arch_atomic64_dec_if_positive(atomic64_t *v)
> "0: lr.d %[p], %[c]\n"
> " addi %[rc], %[p], -1\n"
> " bltz %[rc], 1f\n"
> - " sc.d.rl %[rc], %[rc], %[c]\n"
> + " sc.d.aqrl %[rc], %[rc], %[c]\n"
> " bnez %[rc], 0b\n"
> - " fence rw, rw\n"
> "1:\n"
> : [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
> :
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index 1af8db92250b..9269fceb86e0 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -307,9 +307,8 @@
> __asm__ __volatile__ ( \
> "0: lr.w %0, %2\n" \
> " bne %0, %z3, 1f\n" \
> - " sc.w.rl %1, %z4, %2\n" \
> + " sc.w.aqrl %1, %z4, %2\n" \
> " bnez %1, 0b\n" \
> - " fence rw, rw\n" \
> "1:\n" \
> : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> : "rJ" ((long)__old), "rJ" (__new) \
> @@ -319,9 +318,8 @@
> __asm__ __volatile__ ( \
> "0: lr.d %0, %2\n" \
> " bne %0, %z3, 1f\n" \
> - " sc.d.rl %1, %z4, %2\n" \
> + " sc.d.aqrl %1, %z4, %2\n" \
> " bnez %1, 0b\n" \
> - " fence rw, rw\n" \
> "1:\n" \
> : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> : "rJ" (__old), "rJ" (__new) \

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