linux-kernel - Re: [PATCH] alpha: spinlock: don't perform memory access in locked critical section

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Message-ID: <CAEdQ38HnWbUB=ZNCZeYz8GZH3U4bv_Uag8WL2zT0D=GMcoT+2A@mail.gmail.com>
Date:	Mon, 6 May 2013 13:19:51 -0700
From:	Matt Turner <mattst88@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
	Richard Henderson <rth@...ddle.net>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>
Subject: Re: [PATCH] alpha: spinlock: don't perform memory access in locked
 critical section

On Mon, May 6, 2013 at 1:01 PM, Will Deacon <will.deacon@....com> wrote:
> The Alpha Architecture Reference Manual states that any memory access
> performed between an LD_xL and a STx_C instruction may cause the
> store-conditional to fail unconditionally and, as such, `no useful
> program should do this'.
>
> Linux is a useful program, so fix up the Alpha spinlock implementation
> to use logical operations rather than load-address instructions for
> generating immediates.
>
> Cc: Richard Henderson <rth@...ddle.net>
> Cc: Ivan Kokshaysky <ink@...assic.park.msu.ru>
> Cc: Matt Turner <mattst88@...il.com>
> Signed-off-by: Will Deacon <will.deacon@....com>
> ---
>  arch/alpha/include/asm/spinlock.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/arch/alpha/include/asm/spinlock.h b/arch/alpha/include/asm/spinlock.h
> index 3bba21e..0c357cd 100644
> --- a/arch/alpha/include/asm/spinlock.h
> +++ b/arch/alpha/include/asm/spinlock.h
> @@ -29,7 +29,7 @@ static inline void arch_spin_lock(arch_spinlock_t * lock)
>         __asm__ __volatile__(
>         "1:     ldl_l   %0,%1\n"
>         "       bne     %0,2f\n"
> -       "       lda     %0,1\n"
> +       "       mov     1,%0\n"
>         "       stl_c   %0,%1\n"
>         "       beq     %0,2f\n"
>         "       mb\n"
> @@ -86,7 +86,7 @@ static inline void arch_write_lock(arch_rwlock_t *lock)
>         __asm__ __volatile__(
>         "1:     ldl_l   %1,%0\n"
>         "       bne     %1,6f\n"
> -       "       lda     %1,1\n"
> +       "       mov     1,%1\n"
>         "       stl_c   %1,%0\n"
>         "       beq     %1,6f\n"
>         "       mb\n"
> @@ -106,7 +106,7 @@ static inline int arch_read_trylock(arch_rwlock_t * lock)
>
>         __asm__ __volatile__(
>         "1:     ldl_l   %1,%0\n"
> -       "       lda     %2,0\n"
> +       "       mov     0,%2\n"
>         "       blbs    %1,2f\n"
>         "       subl    %1,2,%2\n"
>         "       stl_c   %2,%0\n"
> @@ -128,9 +128,9 @@ static inline int arch_write_trylock(arch_rwlock_t * lock)
>
>         __asm__ __volatile__(
>         "1:     ldl_l   %1,%0\n"
> -       "       lda     %2,0\n"
> +       "       mov     0,%2\n"
>         "       bne     %1,2f\n"
> -       "       lda     %2,1\n"
> +       "       mov     1,%2\n"
>         "       stl_c   %2,%0\n"
>         "       beq     %2,6f\n"
>         "2:     mb\n"
> --
> 1.8.2.2

I'm not sure of the interpretation that LDA counts as a memory access.

The manual says it's Ra <- Rbv + SEXT(disp).

It's not touching memory that I can see.

Does this fix a known problem or is it just something that you noticed?

Matt
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