Re: [PATCH] Use CAS instead of atomic swap to implement spinlock

[Rich Felker <dalias@libc.org>]

On Wed, Apr 15, 2015 at 01:44:53AM +0300, Alexander Monakov wrote:
> This should allow spinning without constantly dirtying cache lines holding the
> spinlock value.  On architectures without native atomic swap, musl implement
> a_swap by looping around a_cas.
> ---
> If I'm not mistaken this was also suggested by nsz on IRC.
> 
>  src/thread/pthread_spin_lock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/thread/pthread_spin_lock.c b/src/thread/pthread_spin_lock.c
> index df575f0..dabcb31 100644
> --- a/src/thread/pthread_spin_lock.c
> +++ b/src/thread/pthread_spin_lock.c
> @@ -2,6 +2,6 @@
>  
>  int pthread_spin_lock(pthread_spinlock_t *s)
>  {
> -	while (a_swap(s, 1)) a_spin();
> +	while (a_cas(s, 0, 1)) a_spin();
>  	return 0;
>  }

Would it perhaps be better to do something like this?

while (*(volatile int *)s || a_cas(s, 0, 1)) a_spin();

Rich