On 09/01/2016 06:41 PM, Peter Zijlstra wrote:
On Thu, Sep 01, 2016 at 04:30:39PM +0100, Will Deacon wrote:
On Thu, Sep 01, 2016 at 05:27:52PM +0200, Manfred Spraul wrote:
Since spin_unlock_wait() is defined as equivalent to spin_lock();
spin_unlock(), the memory barrier before spin_unlock_wait() is
also not required.
Note that ACQUIRE+RELEASE isn't a barrier.
Both are semi-permeable and things can cross in the middle, like:
x = 1;
LOCK
UNLOCK
r = y;
can (validly) get re-ordered like:
LOCK
r = y;
x = 1;
UNLOCK
So if you want things ordered, as I think you do, I think the smp_mb()
is still needed.
CPU1:
x=1; /* without WRITE_ONCE */
LOCK(l);
UNLOCK(l);
<do_semop>
smp_store_release(x,0)
CPU2;
LOCK(l)
if (smp_load_acquire(x)==1) goto slow_path
<do_semop>
UNLOCK(l)
Ordering is enforced because both CPUs access the same lock.
x=1 can't be reordered past the UNLOCK(l), I don't see that further
guarantees are necessary.
Correct?
--
Manfred
