Re: softlockups in multi_cpu_stop

[Jason Low <jason.low2@hp.com>]

On Sat, 2015-03-07 at 13:54 +0800, Ming Lei wrote:
> On Sat, Mar 7, 2015 at 12:31 PM, Jason Low <jason.low2@hp.com> wrote:
> > On Fri, 2015-03-06 at 13:12 -0800, Jason Low wrote:
> > Cc: Ming Lei <ming.lei@canonical.com>
> > Cc: Davidlohr Bueso <dave@stgolabs.net>
> > Signed-off-by: Jason Low <jason.low2@hp.com>
> 
> Reported-and-tested-by: Ming Lei <ming.lei@canonical.com>

Thanks!

> >  static noinline
> >  bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
> >  {
> >         long count;
> >
> >         rcu_read_lock();
> > -       while (owner_running(sem, owner)) {
> > -               /* abort spinning when need_resched */
> > -               if (need_resched()) {
> > +       while (sem->owner == owner) {
> > +               /*
> > +                * Ensure we emit the owner->on_cpu, dereference _after_
> > +                * checking sem->owner still matches owner, if that fails,
> > +                * owner might point to free()d memory, if it still matches,
> > +                * the rcu_read_lock() ensures the memory stays valid.
> > +                */
> > +               barrier();
> > +
> > +               /* abort spinning when need_resched or owner is not running */
> > +               if (!owner->on_cpu || need_resched()) {
> 
> BTW, could the need_resched() be handled in loop of
> rwsem_optimistic_spin() directly? Then code may get
> simplified a bit.

We still need the need_resched() check here, since if the thread needs
to reschedule, it should immediately stop spinning for the lock.
Otherwise, it could potentially spin for a long time before it checks
for it needs to reschedule.