Re: [PATCH 2/6] locking/rwsem: Document barrier need when waking tasks

[Davidlohr Bueso <dave@stgolabs.net>]

On Tue, 2015-01-27 at 18:07 +0100, Peter Zijlstra wrote:
> On Sun, Jan 25, 2015 at 11:36:05PM -0800, Davidlohr Bueso wrote:
> > The need for the smp_mb in __rwsem_do_wake should be
> > properly documented. Applies to both xadd and spinlock
> > variants.
> > 
> > Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> > ---
> >  kernel/locking/rwsem-spinlock.c | 5 +++++
> >  kernel/locking/rwsem-xadd.c     | 5 +++++
> >  2 files changed, 10 insertions(+)
> > 
> > diff --git a/kernel/locking/rwsem-spinlock.c b/kernel/locking/rwsem-spinlock.c
> > index 2555ae1..54f7a17 100644
> > --- a/kernel/locking/rwsem-spinlock.c
> > +++ b/kernel/locking/rwsem-spinlock.c
> > @@ -85,6 +85,11 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
> >  
> >  		list_del(&waiter->list);
> >  		tsk = waiter->task;
> > +		/*
> > +		 * Ensure that all cores see the read before
> > +		 * setting it to the waiter task to nil, as that
> > +		 * grants the read lock to the next task.
> > +		 */
> >  		smp_mb();
> >  		waiter->task = NULL;
> >  		wake_up_process(tsk);
> 
> Could you enhance that comment by pointing at the pairing code? Is that
> the wait loop in rwsem_down_read_failed()?

Yep.

> Also, the comment confuses, how can all cores observe a read into a
> local variable?

Yep, I'll rephrase that ;)