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

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Date:	Tue, 27 Jan 2015 12:30:27 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Michel Lespinasse <walken@...gle.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] locking/rwsem: Document barrier need when waking
 tasks

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@...e.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 ;)

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