From 24842e6c92d0cc9347ab78132cedddb7b6210640 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 19 Oct 2021 02:08:09 +0200
Subject: [PATCH] rcu/nocb: Make rcu_core() callbacks acceleration preempt-safe
References: bnc#1189998 (PREEMPT_RT prerequisite backports)
Patch-mainline: v5.17-rc0
Git-commit: 24ee940d89277602147ce1b8b4fd87b01b9a6660
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 <valentin.schneider@arm.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
kernel/rcu/tree.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 21518c737c3b..f3d00af10202 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2287,7 +2287,6 @@ 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());
@@ -2314,8 +2313,10 @@ 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);