Messages in this thread Patch in this message | | | From | Qi Zheng <> | Subject | [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put() completes | Date | Thu, 7 Apr 2022 18:33:35 +0800 |
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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@bytedance.com> --- 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
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