Re: wake_q memory ordering

[Peter Zijlstra]

On Thu, Oct 10, 2019 at 02:13:47PM +0200, Manfred Spraul wrote:
> Hi Peter,
> 
> On 10/10/19 1:42 PM, Peter Zijlstra wrote:
> > On Thu, Oct 10, 2019 at 12:41:11PM +0200, Manfred Spraul wrote:
> > > Hi,
> > > 
> > > Waiman Long noticed that the memory barriers in sem_lock() are not really
> > > documented, and while adding documentation, I ended up with one case where
> > > I'm not certain about the wake_q code:
> > > 
> > > Questions:
> > > - Does smp_mb__before_atomic() + a (failed) cmpxchg_relaxed provide an
> > >    ordering guarantee?
> > Yep. Either the atomic instruction implies ordering (eg. x86 LOCK
> > prefix) or it doesn't (most RISC LL/SC), if it does,
> > smp_mb__{before,after}_atomic() are a NO-OP and the ordering is
> > unconditinoal, if it does not, then smp_mb__{before,after}_atomic() are
> > unconditional barriers.
> 
> And _relaxed() differs from "normal" cmpxchg only for LL/SC architectures,
> correct?

Indeed.

> Therefore smp_mb__{before,after}_atomic() may be combined with
> cmpxchg_relaxed, to form a full memory barrier, on all archs.

Just so.

> > > - Is it ok that wake_up_q just writes wake_q->next, shouldn't
> > >    smp_store_acquire() be used? I.e.: guarantee that wake_up_process()
> > >    happens after cmpxchg_relaxed(), assuming that a failed cmpxchg_relaxed
> > >    provides any ordering.
> > There is no such thing as store_acquire, it is either load_acquire or
> > store_release. But just like how we can write load-aquire like
> > load+smp_mb(), so too I suppose we could write store-acquire like
> > store+smp_mb(), and that is exactly what is there (through the implied
> > barrier of wake_up_process()).
> 
> Thanks for confirming my assumption:
> The code is correct, due to the implied barrier inside wake_up_process().

It has a comment there, trying to state this.

		task->wake_q.next = NULL;

		/*
		 * wake_up_process() executes a full barrier, which pairs with
		 * the queueing in wake_q_add() so as not to miss wakeups.
		 */
		wake_up_process(task);