Request for backport fd6bc19d7676 to 4.14 and 4.19 branch

Hi Greg,

We recently hit a hung task timeout issue in synchronize_rcu_expedited on 4.14 branch. The issue seems to be identical to the one described in `fd6bc19d7676 rcu: Fix missed wakeup of exp_wq waiters` Can we backport it to 4.14 and 4.19 branch? The patch doesn't apply cleanly, but it should be trivial to resolve, just do this

- wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rsp->expedited_sequence) & 0x3]); + wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);

I don't know if we should do it for 4.9, because the handling of sequence number is a bit different.

Thanks, David