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Subject[PATCH 01/13] rcu/nocb: Fix potential missed nocb_timer rearm
Date
Two situations can cause a missed nocb timer rearm:

1) rdp(CPU A) queues its nocb timer. The grace period elapses before
the timer get a chance to fire. The nocb_gp kthread is awaken by
rdp(CPU B). The nocb_cb kthread for rdp(CPU A) is awaken and
process the callbacks, again before the nocb_timer for CPU A get a
chance to fire. rdp(CPU A) queues a callback and wakes up nocb_gp
kthread, cancelling the pending nocb_timer without resetting the
corresponding nocb_defer_wakeup.

2) 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.

On both situations, 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.

Case 1) is very unlikely due to timing constraint (the timer fires after
1 jiffy) but still possible in theory. Case 2) is more likely to happen.
But in any case such scenario require the CPU to spend a long time
within a kernel thread without exiting to idle or user space, which is
a pretty exotic behaviour.

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

Fixes: d1b222c6be1f (rcu/nocb: Add bypass callback queueing)
Cc: Stable <stable@vger.kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/rcu/tree_plugin.h | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 2ec9d7f55f99..dd0dc66c282d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1720,7 +1720,11 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force,
rcu_nocb_unlock_irqrestore(rdp, flags);
return false;
}
- del_timer(&rdp->nocb_timer);
+
+ if (READ_ONCE(rdp->nocb_defer_wakeup) > RCU_NOCB_WAKE_NOT) {
+ WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
+ del_timer(&rdp->nocb_timer);
+ }
rcu_nocb_unlock_irqrestore(rdp, flags);
raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
if (force || READ_ONCE(rdp_gp->nocb_gp_sleep)) {
@@ -2349,7 +2353,6 @@ static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp)
return false;
}
ndw = READ_ONCE(rdp->nocb_defer_wakeup);
- WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
ret = wake_nocb_gp(rdp, ndw == RCU_NOCB_WAKE_FORCE, flags);
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DeferredWake"));

--
2.25.1
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