Re: FAILED: patch "[PATCH] rcu: Do RCU GP kthread self-wakeup from softirq and interrupt" failed to apply to 5.0-stable tree

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Wed, Mar 20, 2019 at 06:43:02PM +0100, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 5.0-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.

Hello, Greg,

"This patch does not Cc stable because it does not apply cleanly to
earlier kernel versions."  ;-)

Bo, Jin, Jie, would any of you be interested in doing the backports?
I would of course be happy to review them.

							Thanx, Paul

> ------------------ original commit in Linus's tree ------------------
> 
> >From 1d1f898df6586c5ea9aeaf349f13089c6fa37903 Mon Sep 17 00:00:00 2001
> From: "Zhang, Jun" <jun.zhang@intel.com>
> Date: Tue, 18 Dec 2018 06:55:01 -0800
> Subject: [PATCH] rcu: Do RCU GP kthread self-wakeup from softirq and interrupt
> 
> 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.
> 
> This patch does not Cc stable because it does not apply cleanly to
> earlier kernel versions.
> 
> Fixes: 48a7639ce80c ("rcu: Make callers awaken grace-period kthread")
> Reported-by: "He, Bo" <bo.he@intel.com>
> Co-developed-by: "Zhang, Jun" <jun.zhang@intel.com>
> Co-developed-by: "He, Bo" <bo.he@intel.com>
> Co-developed-by: "xiao, jin" <jin.xiao@intel.com>
> Co-developed-by: Bai, Jie A <jie.a.bai@intel.com>
> Signed-off: "Zhang, Jun" <jun.zhang@intel.com>
> Signed-off: "He, Bo" <bo.he@intel.com>
> Signed-off: "xiao, jin" <jin.xiao@intel.com>
> Signed-off: Bai, Jie A <jie.a.bai@intel.com>
> Signed-off-by: "Zhang, Jun" <jun.zhang@intel.com>
> [ paulmck: Switch from !in_softirq() to "!in_interrupt() &&
>   !in_serving_softirq() to avoid redundant wakeups and to also handle the
>   interrupt-handler scenario as well as the softirq-handler scenario that
>   actually occurred in testing. ]
> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> Link: https://lkml.kernel.org/r/CD6925E8781EFD4D8E11882D20FC406D52A11F61@SHSMSX104.ccr.corp.intel.com
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 9ceb93f848cd..21775eebb8f0 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1593,15 +1593,23 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
>  }
>  
>  /*
> - * Awaken the grace-period kthread.  Don't do a self-awaken, and don't
> - * bother awakening when there is nothing for the grace-period kthread
> - * to do (as in several CPUs raced to awaken, and we lost), and finally
> - * don't try to awaken a kthread that has not yet been created.  If
> - * all those checks are passed, track some debug information and awaken.
> + * Awaken the grace-period kthread.  Don't do a self-awaken (unless in
> + * an interrupt or softirq handler), and don't bother awakening when there
> + * is nothing for the grace-period kthread to do (as in several CPUs raced
> + * to awaken, and we lost), and finally don't try to awaken a kthread that
> + * has not yet been created.  If all those checks are passed, track some
> + * debug information and awaken.
> + *
> + * So why do the self-wakeup when in an interrupt or softirq handler
> + * in the grace-period kthread's context?  Because the kthread might have
> + * been interrupted just as it was going to sleep, and just after the final
> + * pre-sleep check of the awaken condition.  In this case, a wakeup really
> + * is required, and is therefore supplied.
>   */
>  static void rcu_gp_kthread_wake(void)
>  {
> -	if (current == rcu_state.gp_kthread ||
> +	if ((current == rcu_state.gp_kthread &&
> +	     !in_interrupt() && !in_serving_softirq()) ||
>  	    !READ_ONCE(rcu_state.gp_flags) ||
>  	    !rcu_state.gp_kthread)
>  		return;
>