Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()

On Wed, Oct 11, 2017 at 05:07:05PM +0100, David Howells wrote:

Paul E. McKenney [off-list ref] wrote:

quoted

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