On Wed, Oct 11, 2017 at 05:07:05PM +0100, David Howells wrote:
> Paul E. McKenney <paul...@linux.vnet.ibm.com> wrote:
>
> > It does not. In most cases, the barriered version would be
> > smp_store_release().
>
> Ummm... Is that good enough? Is:
>
> WRITE_ONCE(x, 1);
> WRITE_ONCE(x, 2);
>
> equivalent to:
>
> smp_store_release(x, 1);
> smp_store_release(x, 2);
>
> if CONFIG_SMP=n?
smp_store_release(&x, 1);
smp_store_release(&x, 2);
But yes, give or take that smp_store_release() potentially disables
more compiler optimizations than does WRITE_ONCE().
> (Consider what happens if an interrupt messes with x).
OK, I will bite... What is your scenario in which an interrupt
gives different results for CONFIG_SMP=n? The barriers
> If it is good enough, should we be using smp_load_acquire() rather than
> READ_ONCE()?
On x86, that might be OK, give or take that smp_load_acquire() potentially
disables more optimizations than does READ_ONCE(). But on ARM, PowerPC,
MIPS, and so on, smp_load_acquire() emits a memory-barrier instruction
and READ_ONCE() does not.
Thanx, Paul
