Re: [PATCH] swait: add missing barrier to swake_up

[Paul E. McKenney]

On Fri, Sep 01, 2017 at 07:55:29PM +1000, Nicholas Piggin wrote:
> On Fri, 1 Sep 2017 11:23:22 +0200
> Andrea Parri <parri.andrea@xxxxxxxxx> wrote:
> 
> > On Fri, Sep 01, 2017 at 04:14:50PM +1000, Nicholas Piggin wrote:
> > > swake_up and swake_up_all test the swaitqueue outside the lock,
> > > but they are missing the barrier that would ensure visibility
> > > of a previous store that sets the wakeup condition with the
> > > load that tests the swaitqueue. This could lead to a lost wakeup
> > > if there is memory reordering. Fix this as prescribed by the
> > > waitqueue_active comments.
> > > 
> > > Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> > > --
> > > I noticed this when chasing down that rcu hang bug (which
> > > turned out to not be anything of the sort). I might be missing
> > > something here and it's safe somehow, but if so then it should
> > > have a comment where it diverges from normal waitqueues.
> > > 
> > > It looks like there's a few callers which are also testing
> > > swait_active before swake_up without a barrier which look wrong,
> > > so I must be missing something but I'm not sure what.  
> > 
> > Hi Nicholas. I noticed
> > 
> >   35a2897c2a306cca344ca5c0b43416707018f434
> >   ("sched/wait: Remove the lockless swait_active() check in swake_up*()")
> > 
> > in tip:locking/core.
> 
> Oh thanks, I missed that. Should be in 4.14/stable IMO.

This might well have been helpful to me -- I had forgotten about that
fix and am testing without it -- and suffering what look to be lost
timeouts/wakeups.  :-/

							Thanx, Paul