In the percpu_ref_call_confirm_rcu(), we call the wake_up_all()
before calling percpu_ref_put(), which will cause the value of
percpu_ref to be unstable when percpu_ref_switch_to_atomic_sync()
returns.
CPU0 CPU1
percpu_ref_switch_to_atomic_sync(&ref)
--> percpu_ref_switch_to_atomic(&ref)
--> percpu_ref_get(ref); /* put after confirmation */
call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu);
percpu_ref_switch_to_atomic_rcu
--> percpu_ref_call_confirm_rcu
--> data->confirm_switch = NULL;
wake_up_all(&percpu_ref_switch_waitq);
/* here waiting to wake up */
wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
(A)percpu_ref_put(ref);
/* The value of &ref is unstable! */
percpu_ref_is_zero(&ref)
(B)percpu_ref_put(ref);
As shown above, assuming that the counts on each cpu add up to 0 before
calling percpu_ref_switch_to_atomic_sync(), we expect that after switching
to atomic mode, percpu_ref_is_zero() can return true. But actually it will
return different values in the two cases of A and B, which is not what
we expected.
Maybe the original purpose of percpu_ref_switch_to_atomic_sync() is
just to ensure that the conversion to atomic mode is completed, but it
should not return with an extra reference count.
Calling wake_up_all() after percpu_ref_put() ensures that the value of
percpu_ref is stable after percpu_ref_switch_to_atomic_sync() returns.
So just do it.
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
lib/percpu-refcount.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index af9302141bcf..b11b4152c8cd 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -154,13 +154,14 @@ static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
data->confirm_switch(ref);
data->confirm_switch = NULL;
- wake_up_all(&percpu_ref_switch_waitq);
if (!data->allow_reinit)
__percpu_ref_exit(ref);
/* drop ref from percpu_ref_switch_to_atomic() */
percpu_ref_put(ref);
+
+ wake_up_all(&percpu_ref_switch_waitq);
}
static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
--
2.20.1
