Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait()

[Chris Metcalf]

On 5/26/2016 10:19 AM, Peter Zijlstra wrote:
> --- a/arch/tile/lib/spinlock_32.c
> +++ b/arch/tile/lib/spinlock_32.c
> @@ -72,10 +72,14 @@ void arch_spin_unlock_wait(arch_spinlock
>   	if (next == curr)
>   		return;
>   
> +	smp_rmb();
> +
>   	/* Wait until the current locker has released the lock. */
>   	do {
>   		delay_backoff(iterations++);
>   	} while (READ_ONCE(lock->current_ticket) == curr);
> +
> +	smp_acquire__after_ctrl_dep();
>   }
>   EXPORT_SYMBOL(arch_spin_unlock_wait);
>   
> --- a/arch/tile/lib/spinlock_64.c
> +++ b/arch/tile/lib/spinlock_64.c
> @@ -72,10 +72,14 @@ void arch_spin_unlock_wait(arch_spinlock
>   	if (arch_spin_next(val) == curr)
>   		return;
>   
> +	smp_rmb();
> +
>   	/* Wait until the current locker has released the lock. */
>   	do {
>   		delay_backoff(iterations++);
>   	} while (arch_spin_current(READ_ONCE(lock->lock)) == curr);
> +
> +	smp_acquire__after_ctrl_dep();
>   }
>   EXPORT_SYMBOL(arch_spin_unlock_wait);

The smp_rmb() are unnecessary for tile.  We READ_ONCE next/curr from the
lock and compare them, so we know the load(s) are complete.  There's no
microarchitectural speculation going on so that's that.  Then we READ_ONCE
the next load on the lock from within the wait loop, so our load/load
ordering is guaranteed.

With that change,

Acked-by: Chris Metcalf <cmetcalf@xxxxxxxxxxxx> [for tile]

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com