[PATCH] percpu_rwsem: fix missed wakeup due to reordering of load

[Prateek Sood]

In a scenario where cpu_hotplug_lock percpu_rw_semaphore is already
acquired for read operation by P1 using percpu_down_read().

Now we have P1 in the path of releaseing the cpu_hotplug_lock and P2
is in the process of acquiring cpu_hotplug_lock.

P1                                               P2
percpu_up_read() path                      percpu_down_write() path

                                          rcu_sync_enter() //gp_state=GP_PASSED

rcu_sync_is_idle() //returns false        down_write(rw_sem)

__percpu_up_read()

[L] task = rcu_dereference(w->task) //NULL

smp_rmb()                                  [S] w->task = current

                                            smp_mb()

                                           [L] readers_active_check() //fails
					     schedule()

[S] __this_cpu_dec(read_count)

Since load of task can result in NULL. This can lead to missed wakeup
in rcuwait_wake_up(). Above sequence violated the following constraint
in rcuwait_wake_up():

	 WAIT                WAKE
[S] tsk = current	  [S] cond = true
MB (A)	                    MB (B)
[L] cond		  [L] tsk

This can happen as smp_rmb() in rcuwait_wake_up() will provide ordering
of load before barrier with load and store after barrier for arm64
architecture. Here the requirement is to order store before smp_rmb()
with load after the smp_rmb().

For the usage of rcuwait_wake_up() in __percpu_up_read() full barrier
(smp_mb) is required to complete the constraint of rcuwait_wake_up().

Signed-off-by: Prateek Sood <prsood@xxxxxxxxxxxxxx>
---
 kernel/exit.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index f1d74f0..a10820d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -306,7 +306,7 @@ void rcuwait_wake_up(struct rcuwait *w)
 	 *        MB (A)	      MB (B)
 	 *    [L] cond		  [L] tsk
 	 */
-	smp_rmb(); /* (B) */
+	smp_mb(); /* (B) */
 
 	/*
 	 * Avoid using task_rcu_dereference() magic as long as we are careful,
-- 
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