Le Thu, Sep 05, 2024 at 08:32:16PM +0200, Frederic Weisbecker a écrit :
> Le Wed, Sep 04, 2024 at 06:52:36AM -0700, Paul E. McKenney a écrit :
> > > Yes, I'm preparing an update for the offending patch (which has one more
> > > embarassing issue while I'm going through it again).
> >
> > Very good, thank you!
>
> So my proposal for a replacement patch is this (to replace the patch
> of the same name in Neeraj tree):
FYI, the diffstat against the previous version of the same patch is as follows.
The rationale being:
1) rdp->nocb_cb_kthread doesn't need to be protected by nocb_gp_kthread_mutex
2) Once rcuoc is parked, we really _must_ observe the callback list counter
decremented
after the barrier's completion.
3) This fixes another issue: rcuoc must be parked _before_
rcu_nocb_queue_toggle_rdp() is called, otherwise a nocb locked sequence
within rcuoc would race with rcuog clearing SEGCBLIST_OFFLOADED concurrently,
leaving the nocb locked forever.
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 755ada098035..97b99cd06923 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1056,6 +1056,13 @@ static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
/* Flush all callbacks from segcblist and bypass */
rcu_barrier();
+ /*
+ * Make sure the rcuoc kthread isn't in the middle of a nocb locked
+ * sequence while offloading is deactivated, along with nocb locking.
+ */
+ if (rdp->nocb_cb_kthread)
+ kthread_park(rdp->nocb_cb_kthread);
+
rcu_nocb_lock_irqsave(rdp, flags);
WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
@@ -1064,13 +1071,11 @@ static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
wake_gp = rcu_nocb_queue_toggle_rdp(rdp);
mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
+
if (rdp_gp->nocb_gp_kthread) {
if (wake_gp)
wake_up_process(rdp_gp->nocb_gp_kthread);
- if (rdp->nocb_cb_kthread)
- kthread_park(rdp->nocb_cb_kthread);
-
swait_event_exclusive(rdp->nocb_state_wq,
rcu_nocb_rdp_deoffload_wait_cond(rdp));
} else {
