On Fri, Jun 19, 2015 at 10:55:26AM +0100, Vineet Gupta wrote:
> A quad core SMP build could get into hardware livelock with concurrent
> LLOCK/SCOND. Workaround that by adding a PREFETCHW which is serialized by
> SCU (System Coherency Unit). It brings the cache line in Exclusive state
> and makes others invalidate their lines. This gives enough time for
> winner to complete the LLOCK/SCOND, before others can get the line back.
>
> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> ---
> arch/arc/include/asm/atomic.h | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h
> index 20b7dc17979e..03484cb4d16d 100644
> --- a/arch/arc/include/asm/atomic.h
> +++ b/arch/arc/include/asm/atomic.h
> @@ -23,13 +23,21 @@
>
> #define atomic_set(v, i) (((v)->counter) = (i))
>
> +#ifdef CONFIG_ISA_ARCV2
> +#define PREFETCHW " prefetchw [%1] \n"
> +#else
> +#define PREFETCHW
> +#endif
> +
> #define ATOMIC_OP(op, c_op, asm_op) \
> static inline void atomic_##op(int i, atomic_t *v) \
> { \
> unsigned int temp; \
> \
> __asm__ __volatile__( \
> - "1: llock %0, [%1] \n" \
> + "1: \n" \
> + PREFETCHW \
> + " llock %0, [%1] \n" \
> " " #asm_op " %0, %0, %2 \n" \
> " scond %0, [%1] \n" \
> " bnz 1b \n" \
Curious, but are you *sure* the prefetch should be *inside* the loop?
On most ll/sc architectures, that's a livelock waiting to happen because
you ping-pong the cache-line around in exclusive state.
Will
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