On Thu, Apr 07, 2022 at 06:33:35PM +0800, Qi Zheng 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. > > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> Are any users affected by this? If so, I think a Fixes tag is necessary. The fix LGTM. Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> Thanks.
