Re: [PATCH 01/16] rcu/nocb: Fix potential missed nocb_timer rearm

On Thu, Jan 28, 2021 at 10:48:34AM -0800, Paul E. McKenney wrote:

On Thu, Jan 28, 2021 at 06:12:07PM +0100, Frederic Weisbecker wrote:

quoted

The "nocb_bypass_timer" ends up calling wake_nocb_gp() which deletes
the pending "nocb_timer" (note they are not the same timers) for the
given rdp without resetting the matching state stored in nocb_defer
wakeup.

As a result, a future call_rcu() on that rdp may be fooled and think the
timer is armed when it's not, missing a deferred nocb_gp wakeup.

Fix this with resetting rdp->nocb_defer_wakeup when we disarm the timer.

Fixes: d1b222c6be1f (rcu/nocb: Add bypass callback queueing)
Cc: Stable <redacted>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <redacted>
Cc: Neeraj Upadhyay <redacted>
Cc: Boqun Feng <redacted>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tree_plugin.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 7e33dae0e6ee..a44f80d7661b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h

@@ -1705,6 +1705,8 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force,
 		rcu_nocb_unlock_irqrestore(rdp, flags);
 		return false;
 	}
+
+	rdp->nocb_defer_wakeup = RCU_NOCB_WAKE_NOT;

Given this change, does it make sense to remove the
setting of ->nocb_defer_wakeup to RCU_NOCB_WAKE_NOT from the
do_nocb_deferred_wakeup_common() function?

I do it later in "[PATCH 09/16] rcu/nocb: Merge nocb_timer to the rdp leader"

Does the above assignment need
to be WRITE_ONCE(), in other words, are all reads of ->nocb_defer_wakeup
done with either ->nocb_lock or ->nocb_gp_lock held?  (I do not believe
that this is the case.)

Ah indeed it should probably be done with WRITE_ONCE() because it's read
locklessly on many places.

Thanks.

							Thanx, Paul

quoted

 	del_timer(&rdp->nocb_timer);
 	rcu_nocb_unlock_irqrestore(rdp, flags);
 	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
-- 
2.25.1