nohz: Allow rcu extended quiescent state handling seperately from tick stop

It is assumed that rcu won't be used once we switch to tickless
mode and until we restart the tick. However this is not always
true, as in x86-64 where we dereference the idle notifiers after
the tick is stopped.

To prepare for fixing this, add two new APIs:
tick_nohz_idle_enter_norcu() and tick_nohz_idle_exit_norcu().

If no use of RCU is made in the idle loop between
tick_nohz_enter_idle() and tick_nohz_exit_idle() calls, the arch
must instead call the new *_norcu() version such that the arch doesn't
need to call rcu_idle_enter() and rcu_idle_exit().

Otherwise the arch must call tick_nohz_enter_idle() and
tick_nohz_exit_idle() and also call explicitly:

- rcu_idle_enter() after its last use of RCU before the CPU is put
to sleep.
- rcu_idle_exit() before the first use of RCU after the CPU is woken
up.

Signed-off-by: Frederic Weisbecker <[email protected]>
Cc: Mike Frysinger <[email protected]>
Cc: Guan Xuetao <[email protected]>
Cc: David Miller <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Hans-Christian Egtvedt <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Russell King <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Paul Mundt <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index cfb657e92849..55d2cf455f63 100644
--- a/arch/um/kernel/process.c
+++ b/arch/um/kernel/process.c
@@ -246,10 +246,10 @@ void default_idle(void)
 		if (need_resched())
 			schedule();
 
-		tick_nohz_idle_enter();
+		tick_nohz_idle_enter_norcu();
 		nsecs = disable_timer();
 		idle_sleep(nsecs);
-		tick_nohz_idle_exit();
+		tick_nohz_idle_exit_norcu();
 	}
 }