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 <[email protected]>
---
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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is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
