On Thu, Apr 07, 2022 at 03:57:52PM -0700, Andrew Morton wrote: > (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 Looks all current users expect the refcount is stable after percpu_ref_switch_to_atomic_sync returns, even though the API itself doesn't mention the point explicitly. > > 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); > > } Looks fine: Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx> Thanks, Ming
