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

From: Prateek Sood
Date: Fri Dec 21 2018 - 02:29:56 EST

P1 is releaseing the cpu_hotplug_lock and P2 is acquiring

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)


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

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


[L] readers_active_check() //fails

[S] __this_cpu_dec(read_count)

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

[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>
Acked-by: Davidlohr Bueso <dbueso@xxxxxxx>

kernel/exit.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index ac1a814..696e0e1 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -298,7 +298,7 @@ void rcuwait_wake_up(struct rcuwait *w)
* Order condition vs @task, such that everything prior to the load
* of @task is visible. This is the condition as to why the user called
- * rcuwait_trywake() in the first place. Pairs with set_current_state()
+ * rcuwait_wake_up() in the first place. Pairs with set_current_state()
* barrier (A) in rcuwait_wait_event().
@@ -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,
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.,
is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.