From 24ee940d89277602147ce1b8b4fd87b01b9a6660 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Tue, 19 Oct 2021 02:08:09 +0200 Subject: rcu/nocb: Make rcu_core() callbacks acceleration preempt-safe While reporting a quiescent state for a given CPU, rcu_core() takes advantage of the freshly loaded grace period sequence number and the locked rnp to accelerate the callbacks whose sequence number have been assigned a stale value. This action is only necessary when the rdp isn't offloaded, otherwise the NOCB kthreads already take care of the callbacks progression. However the check for the offloaded state is volatile because it is performed outside the IRQs disabled section. It's possible for the offloading process to preempt rcu_core() at that point on PREEMPT_RT. This is dangerous because rcu_core() may end up accelerating callbacks concurrently with NOCB kthreads without appropriate locking. Fix this with moving the offloaded check inside the rnp locking section. Reported-and-tested-by: Valentin Schneider Reviewed-by: Valentin Schneider Tested-by: Sebastian Andrzej Siewior Signed-off-by: Thomas Gleixner Cc: Peter Zijlstra Cc: Sebastian Andrzej Siewior Cc: Josh Triplett Cc: Joel Fernandes Cc: Boqun Feng Cc: Neeraj Upadhyay Cc: Uladzislau Rezki Cc: Thomas Gleixner Signed-off-by: Frederic Weisbecker Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) (limited to 'kernel/rcu/tree.c') diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index a329adfece86..5985698f3341 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2278,7 +2278,6 @@ rcu_report_qs_rdp(struct rcu_data *rdp) unsigned long flags; unsigned long mask; bool needwake = false; - const bool offloaded = rcu_rdp_is_offloaded(rdp); struct rcu_node *rnp; WARN_ON_ONCE(rdp->cpu != smp_processor_id()); @@ -2305,8 +2304,10 @@ rcu_report_qs_rdp(struct rcu_data *rdp) /* * This GP can't end until cpu checks in, so all of our * callbacks can be processed during the next GP. + * + * NOCB kthreads have their own way to deal with that. */ - if (!offloaded) + if (!rcu_rdp_is_offloaded(rdp)) needwake = rcu_accelerate_cbs(rnp, rdp); rcu_disable_urgency_upon_qs(rdp); -- cgit v1.2.3