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

Matt Turner
2013-05-06T20:25:31Z

On Mon, May 6, 2013 at 1:01 PM, Will Deacon <will.deacon@arm.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@twiddle.net>
> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
> Cc: Matt Turner <mattst88@gmail.com>
> Signed-off-by: Will Deacon <will.deacon@arm.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