Re: [PATCH V2 11/19] csky: Atomic operations
From: Andrea Parri
Date: Sat Jul 07 2018 - 16:10:45 EST
On Sat, Jul 07, 2018 at 04:08:47PM +0800, Guo Ren wrote:
> On Fri, Jul 06, 2018 at 02:17:16PM +0200, Peter Zijlstra wrote:
> CPU0 CPU1
> WRITE_ONCE(x, 1) WRITE_ONCE(y, 1)
> r0 = xchg(&y, 2) r1 = xchg(&x, 2)
> must not allow: r0==0 && r1==0
> So we must add a smp_mb between WRITE_ONCE() and xchg(), right?
The state (r0==0 && r1==0) _must_ not be allowed in the above snippet (so,
even without the additional smp_mb() between WRITE_ONCE() and xchg()). In
informal terms, xchg() provides the smp_mb().
Compare implementations of xchg() and xchg_relaxed(). The following could
also be helpful (in addition to the references pointed out earlier):
> Guo Ren