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Date: Wed, 25 Sep 2019 23:38:48 +0000
From: Alistair Francis <Alistair.Francis@....com>
To: "musl@...ts.openwall.com" <musl@...ts.openwall.com>, "palmer@...ive.com"
<palmer@...ive.com>
Subject: Re: [PATCH] correct the operand specifiers in the riscv64 CAS
routines
On Tue, 2019-09-24 at 20:30 -0700, Palmer Dabbelt wrote:
> The operand sepcifiers in a_cas and a_casp for riscv64 were
> incorrect:
> there's a backwards branch in the routine, so despite tmp being
> written
> at the end of the assembly fragment it cannot be allocated in one of
> the
> input registers because the input values may be needed for another
> trip
> around the loop.
>
> For code that follows the guarnteed forward progress requirements, he
> backwards branch is rarely taken: SiFive's hardware only fails a
> store
> conditional on execptional cases (ie, instruction cache misses inside
> the loop), and until recently a bug in QEMU allowed back-to-back
> store conditionals to succeed. The bug has been fixed in the latest
> QEMU release, but it turns out that the fix caused this latent bug in
> musl to manifest.
>
> Full disclosure: I haven't actually even compiled musl. I just
> guessed
> this would fix a bug introducted by the new QEMU behavior, Alistair
> (CC'd) actually checked it fixes the problem. The rest is just
> conjecture.
Reviewed-by: Alistair Francis <alistair.francis@....com>
Tested-by: Alistair Francis <alistair.francis@....com>
> ---
> arch/riscv64/atomic_arch.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv64/atomic_arch.h b/arch/riscv64/atomic_arch.h
> index c976534284aa..41ad4d04907c 100644
> --- a/arch/riscv64/atomic_arch.h
> +++ b/arch/riscv64/atomic_arch.h
> @@ -14,7 +14,7 @@ static inline int a_cas(volatile int *p, int t, int
> s)
> " sc.w.aqrl %1, %4, (%2)\n"
> " bnez %1, 1b\n"
> "1:"
> - : "=&r"(old), "=r"(tmp)
> + : "=&r"(old), "=&r"(tmp)
> : "r"(p), "r"(t), "r"(s)
> : "memory");
> return old;
> @@ -31,7 +31,7 @@ static inline void *a_cas_p(volatile void *p, void
> *t, void *s)
> " sc.d.aqrl %1, %4, (%2)\n"
> " bnez %1, 1b\n"
> "1:"
> - : "=&r"(old), "=r"(tmp)
> + : "=&r"(old), "=&r"(tmp)
> : "r"(p), "r"(t), "r"(s)
> : "memory");
> return old;
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