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In-Reply-To: <YmIcyAIXG+JLnU7e@tardis>
From:   Guo Ren <guoren@kernel.org>
Date:   Sun, 24 Apr 2022 15:52:07 +0800
Message-ID: <CAJF2gTQiQMdtRVf2xiD_LavxZVTHgY92d0BPhr47W47vEN0MhA@mail.gmail.com>
Subject: Re: [PATCH V2 0/3] riscv: atomic: Optimize AMO instructions usage
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     Dan Lustig <dlustig@nvidia.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Guo Ren <guoren@linux.alibaba.com>
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On Fri, Apr 22, 2022 at 11:11 AM Boqun Feng <boqun.feng@gmail.com> 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@gmail.com> 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@nvidia.com> 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@nvidia.com> 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@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
> > > > > >>
> > > > > >> 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@linux.alibaba.com>
> > > > 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@linux.alibaba.com>
> > > > Signed-off-by: Guo Ren <guoren@kernel.org>
> > > > Cc: Palmer Dabbelt <palmer@dabbelt.com>
> > > > Cc: Mark Rutland <mark.rutland@arm.com>
> > > > Cc: Andrea Parri <parri.andrea@gmail.com>
> > > > Cc: Dan Lustig <dlustig@nvidia.com>
> > > > Cc: stable@vger.kernel.org
> > > > ---
> > > >  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/