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Date: Sun, 22 May 2022 21:12:56 +0800
From: Guo Ren <guoren@...nel.org>
To: Palmer Dabbelt <palmer@...belt.com>
Cc: Andrea Parri <parri.andrea@...il.com>,
Arnd Bergmann <arnd@...db.de>,
Mark Rutland <mark.rutland@....com>,
Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Boqun Feng <boqun.feng@...il.com>,
Daniel Lustig <dlustig@...dia.com>,
linux-arch <linux-arch@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-riscv <linux-riscv@...ts.infradead.org>,
Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH V4 5/5] riscv: atomic: Optimize LRSC-pairs atomic ops with
.aqrl annotation
On Sun, May 22, 2022 at 4:46 AM Palmer Dabbelt <palmer@...belt.com> wrote:
>
> On Wed, 04 May 2022 20:55:26 PDT (-0700), guoren@...nel.org wrote:
> > From: Guo Ren <guoren@...ux.alibaba.com>
> >
> > 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"
> 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.
> 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@...ux.alibaba.com>
> > Signed-off-by: Guo Ren <guoren@...nel.org>
> > Cc: Palmer Dabbelt <palmer@...belt.com>
> > Cc: Mark Rutland <mark.rutland@....com>
> > Cc: Dan Lustig <dlustig@...dia.com>
> > Cc: Andrea Parri <parri.andrea@...il.com>
> > ---
> > 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) \
--
Best Regards
Guo Ren
ML: https://lore.kernel.org/linux-csky/
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