[PATCH tip/core/rcu 10/40] rcu: Make rcu_note_context_switch() do deferred NOCB wakeups

[Paul E. McKenney]

If a CONFIG_RCU_NOCB_CPUS kernel invokes call_rcu() with interrupts
disabled, wakeups must be deferred in order to avoid self-deadlock in the
cases where the disabled interrupts are due to scheduler locks being held.
In this case, a flag is set and is checked on entry to extended quiescent
states (usermode, idle), on exit from the RCU_SOFTIRQ handler, when the
CPU in question goes offline, on a subsequent invocation of call_rcu(),
and from rcu_all_qs().  However, a given CPU could avoid all of those
states for a considerable length of time.

This commit therefore allows an invocation of rcu_note_context_switch()
to do the wakeup.  It also makes the wakeup function clear the
deferred-wakeup flag.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index eaa2b6d7abe2..7fa46967021f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -495,12 +495,16 @@ static void rcu_momentary_dyntick_idle(void)
  */
 void rcu_note_context_switch(void)
 {
+	struct rcu_state *rsp;
+
 	barrier(); /* Avoid RCU read-side critical sections leaking down. */
 	trace_rcu_utilization(TPS("Start context switch"));
 	rcu_sched_qs();
 	rcu_preempt_note_context_switch();
 	if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
 		rcu_momentary_dyntick_idle();
+	for_each_rcu_flavor(rsp)
+		do_nocb_deferred_wakeup(this_cpu_ptr(rsp->rda));
 	trace_rcu_utilization(TPS("End context switch"));
 	barrier(); /* Avoid RCU read-side critical sections leaking up. */
 }
-- 
2.5.2