On Thu, Aug 02, 2018 at 07:08:41PM +0000, David Chen wrote:
> Hi all,
>
> We'd like to have the following commit backport to 4.9 branch to fix an
> issue we are seeing.
>
> 35a2897c2a306cca344ca5c0b43416707018f434
> sched/wait: Remove the lockless swait_active() check in swake_up*()
>
> In 4.9 branch, we hit an issue in RCU, where the NOCB follower list not getting
> reclaimed and causing OOM.
>
> In discussion with Paul, we were able to figure out the problem was because of
> missed wake up resulted from lack of proper memory barrier between setting
> wake up condition and swake_up().
>
> nocb_leader_wait()
> {
> *tail = rdp->nocb_gp_head;
> smp_mb__after_atomic(); /* Store *tail before wakeup. */
> if (rdp != my_rdp && tail == &rdp->nocb_follower_head) {
> swake_up(&rdp->nocb_wq);
>
> Note, that the smp_mb__after_atomic() is only a compiler barrier on x86.
> Originally I was going to change the barrier to smp_mb(). But then I found out
> master has the above mentioned patch that solves the same class of problem by
> removing the lockless check inside swake_up().
>
> So I'm wonder if we can backport this patch to 4.9 branch to solve this issue,
> and maybe solve other potential missed wake up issue as well.
Now applied, thanks.
greg k-h
