[tip: core/rcu] rcu: Ensure that ->rcu_urgent_qs is set before resched IPI

[tip-bot2 for Joel Fernandes (Google)]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     05ef9e9eb3dade21413680f41eb0170778e8ae2b
Gitweb:        https://git.kernel.org/tip/05ef9e9eb3dade21413680f41eb0170778e8ae2b
Author:        Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
AuthorDate:    Thu, 15 Aug 2019 22:59:14 -04:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Wed, 30 Oct 2019 08:34:35 -07:00

rcu: Ensure that ->rcu_urgent_qs is set before resched IPI

The RCU-specific resched_cpu() function sends a resched IPI to the
specified CPU, which can be used to force the tick on for a given
nohz_full CPU.  This is needed when this nohz_full CPU is looping in the
kernel while blocking the current grace period.  However, for the tick
to actually be forced on in all cases, that CPU's rcu_data structure's
->rcu_urgent_qs flag must be set beforehand.  This commit therefore
causes rcu_implicit_dynticks_qs() to set this flag prior to invoking
resched_cpu() on a holdout nohz_full CPU.

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8110514..0d83b19 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1073,6 +1073,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	if (tick_nohz_full_cpu(rdp->cpu) &&
 		   time_after(jiffies,
 			      READ_ONCE(rdp->last_fqs_resched) + jtsq * 3)) {
+		WRITE_ONCE(*ruqp, true);
 		resched_cpu(rdp->cpu);
 		WRITE_ONCE(rdp->last_fqs_resched, jiffies);
 	}