lkml.org 
[lkml]   [2017]   [Sep]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] swait: add missing barrier to swake_up
On Fri, 1 Sep 2017 11:23:22 +0200
Andrea Parri <parri.andrea@gmail.com> 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@gmail.com>
> > --
> > 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.

As I said as well, several callers have picked up bad habits. RCU
looks okay though.

Thanks,
Nick

\
 
 \ /
  Last update: 2017-09-01 11:58    [W:0.049 / U:5.048 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site