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@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8110514..0d83b19 100644 (file)
--- 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);
        }