[tip: core/rcu] stop_machine: Provide RCU quiescent state in multi_cpu_stop()

["tip-bot2 for Paul E. McKenney" <tip-bot2@xxxxxxxxxxxxx>]

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

Commit-ID:     366237e7b0833faa2d8da7a8d7d7da8c3ca802e5
Gitweb:        https://git.kernel.org/tip/366237e7b0833faa2d8da7a8d7d7da8c3ca802e5
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Wed, 10 Jul 2019 08:01:01 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Sat, 05 Oct 2019 10:46:05 -07:00

stop_machine: Provide RCU quiescent state in multi_cpu_stop()

When multi_cpu_stop() loops waiting for other tasks, it can trigger an RCU
CPU stall warning.  This can be misleading because what is instead needed
is information on whatever task is blocking multi_cpu_stop().  This commit
therefore inserts an RCU quiescent state into the multi_cpu_stop()
function's waitloop.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 include/linux/rcutree.h | 1 +
 kernel/rcu/tree.c       | 2 +-
 kernel/stop_machine.c   | 1 +
 3 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 18b1ed9..c5147de 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -37,6 +37,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
 
 void rcu_barrier(void);
 bool rcu_eqs_special_set(int cpu);
+void rcu_momentary_dyntick_idle(void);
 unsigned long get_state_synchronize_rcu(void);
 void cond_synchronize_rcu(unsigned long oldstate);
 
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 238f93b..a5c296d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -364,7 +364,7 @@ bool rcu_eqs_special_set(int cpu)
  *
  * The caller must have disabled interrupts and must not be idle.
  */
-static void __maybe_unused rcu_momentary_dyntick_idle(void)
+void rcu_momentary_dyntick_idle(void)
 {
 	int special;
 
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index c7031a2..34c4f11 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -233,6 +233,7 @@ static int multi_cpu_stop(void *data)
 			 */
 			touch_nmi_watchdog();
 		}
+		rcu_momentary_dyntick_idle();
 	} while (curstate != MULTI_STOP_EXIT);
 
 	local_irq_restore(flags);