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Message-ID: <Ylu0GqNmYmCnpv9Z@tardis>
Date: Sun, 17 Apr 2022 14:30:50 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Guo Ren <guoren@...nel.org>
Cc: Andrea Parri <parri.andrea@...il.com>,
Daniel Lustig <dlustig@...dia.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Arnd Bergmann <arnd@...db.de>,
Palmer Dabbelt <palmer@...belt.com>,
Mark Rutland <mark.rutland@....com>,
Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
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 V2 0/3] riscv: atomic: Optimize AMO instructions usage
On Sun, Apr 17, 2022 at 12:51:38PM +0800, Guo Ren wrote:
> Hi Boqun & Andrea,
>
> On Sun, Apr 17, 2022 at 10:26 AM Boqun Feng <boqun.feng@...il.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.
>
Thanks for the confirmation, btw, where can I find the RVWMO spec?
> 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.
>
Note that Linux kernel doesn't require cmpxchg() to provide any order if
cmpxchg() fails to update the memory location. So you won't need to
strengthen the atomic_cmpxchg().
Regards,
Boqun
> >
> > Regards,
> > Boqun
>
>
>
> --
> Best Regards
> Guo Ren
>
> ML: https://lore.kernel.org/linux-csky/
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