Re: [PATCH] rcu: Make nocb leader kthreads process pending callbacks after spawning

Paul E. McKenney
2014-08-28T12:58:46Z

On Thu, Aug 28, 2014 at 11:16:49AM +0530, Amit Shah wrote:
> On (Wed) 27 Aug 2014 [16:43:40], Pranith Kumar wrote:
> > The nocb callbacks generated before the nocb kthreads are spawned are
> > enqueued in the nocb queue for later processing. Commit fbce7497ee5af ("rcu:
> > Parallelize and economize NOCB kthread wakeups") introduced nocb leader kthreads
> > which checked the nocb_leader_wake flag to see if there were any such pending
> > callbacks. A case was reported in which newly spawned leader kthreads were not
> > processing the pending callbacks as this flag was not set, which led to a boot
> > hang.
> >
> > The following commit ensures that the newly spawned nocb kthreads process the
> > pending callbacks by allowing the kthreads to run immediately after spawning
> > instead of waiting. This is done by inverting the logic of nocb_leader_wake
> > tests to nocb_leader_sleep which allows us to use the default initialization of
> > this flag to 0 to let the kthreads run.
> >
> > Reported-by: Amit Shah <amit.shah@redhat.com>
> > Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> > Link: http://www.spinics.net/lists/kernel/msg1802899.html
> > ---
> > kernel/rcu/tree.h | 2 +-
> > kernel/rcu/tree_plugin.h | 24 ++++++++++++------------
> > 2 files changed, 13 insertions(+), 13 deletions(-)
>
> I'd have split this into two patches: one for the variable rename and
> one for fixing the bug.
>
> However, the backport Paul posted does work fine for me on master, so
> you can add my
>
> Tested-by: Amit Shah <amit.shah@redhat.com>

Thank you again, Amit, both for finding this problem and for your
testing efforts!

Thanx, Paul