(cc Ming Lei) On Thu, 7 Apr 2022 18:33:35 +0800 Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote: > 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. Thanks. I'll grab this, but shall await input from others before doing anything else with it. > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> > +++ 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)
