Re: FAILED: patch "[PATCH] rcu: Do RCU GP kthread self-wakeup from softirq and interrupt" failed to apply to 4.19-stable tree

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

On Thu, Mar 21, 2019 at 03:31:28PM +0000, He, Bo wrote:
> The rcu_gp_kthread_wake() function is invoked when it might be necessary
> to wake the RCU grace-period kthread.  Because self-wakeups are normally
> a useless waste of CPU cycles, if rcu_gp_kthread_wake() is invoked from
> this kthread, it naturally refuses to do the wakeup.
> 
> Unfortunately, natural though it might be, this heuristic fails when
> rcu_gp_kthread_wake() is invoked from an interrupt or softirq handler
> that interrupted the grace-period kthread just after the final check of
> the wait-event condition but just before the schedule() call.  In this
> case, a wakeup is required, even though the call to rcu_gp_kthread_wake()
> is within the RCU grace-period kthread's context.  Failing to provide
> this wakeup can result in grace periods failing to start, which in turn
> results in out-of-memory conditions.
> 
> This race window is quite narrow, but it actually did happen during real
> testing.  It would of course need to be fixed even if it was strictly
> theoretical in nature.
> 
> [ backport for 4.19 commit 1d1f898df6586c5ea9aeaf349f13089c6fa37903
> upstream. ]

Thanks for all of these, now queued up.

greg k-h