[PATCH] rcu: Fix missed wakeup of exp_wq waiters

[Neeraj Upadhyay]

For the tasks waiting in exp_wq inside exp_funnel_lock(),
there is a chance that they might be indefinitely blocked
in below scenario:

1. There is a task waiting on exp sequence 0b'100' inside
   exp_funnel_lock().

   _synchronize_rcu_expedited()
     s = 0b'100
     exp_funnel_lock()
       wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3]

2. The Exp GP completes and task (task1) holding exp_mutex queues
   worker and schedules out.

   _synchronize_rcu_expedited()
     s = 0b'100
     queue_work(rcu_gp_wq, &rew.rew_work)
       wake_up_worker()
         schedule()

3. kworker A picks up the queued work and completes the exp gp
   sequence.

   rcu_exp_wait_wake()
     rcu_exp_wait_wake()
       rcu_exp_gp_seq_end(rsp) // rsp->expedited_sequence is incremented
                               // to 0b'100'

4. task1 does not enter wait queue, as sync_exp_work_done() returns true,
   and releases exp_mutex.

   wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
     sync_exp_work_done(rsp, s));
   mutex_unlock(&rsp->exp_mutex);

5. Next exp GP completes, and sequence number is incremented:

   rcu_exp_wait_wake()
     rcu_exp_wait_wake()
       rcu_exp_gp_seq_end(rsp) // rsp->expedited_sequence = 0b'200'

6. As kworker A uses current expedited_sequence, it wakes up workers
   from wrong wait queue index - it should have worken wait queue
   corresponding to 0b'100' sequence, but wakes up the ones for
   0b'200' sequence. This results in task at step 1 indefinitely blocked.

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

Fix this by using the correct sequence number for wake_up_all() inside
rcu_exp_wait_wake().

Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
---
 kernel/rcu/tree_exp.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index e4b77d3..28979d3 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -557,7 +557,7 @@ static void rcu_exp_wait_wake(unsigned long s)
 			spin_unlock(&rnp->exp_lock);
 		}
 		smp_mb(); /* All above changes before wakeup. */
-		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]);
+		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);
 	}
 	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
 	mutex_unlock(&rcu_state.exp_wake_mutex);
-- 
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