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Date:   Tue, 24 Jan 2017 13:51:20 -0800
From:   "Paul E. McKenney" <>
        "Paul E. McKenney" <>,
        Petr Mladek <>
Subject: [PATCH v3 tip/core/rcu 09/18] rcu: Once again use NMI-based stack traces in stall warnings

This commit is for all intents and purposes a revert of bc1dce514e9b
("rcu: Don't use NMIs to dump other CPUs' stacks").  The reason to suppose
that this can now safely be reverted is the presence of 42a0bb3f7138
("printk/nmi: generic solution for safe printk in NMI"), which is said
to have made NMI-based stack dumps safe.

However, this reversion keeps one nice property of bc1dce514e9b
("rcu: Don't use NMIs to dump other CPUs' stacks"), namely that
only those CPUs blocking the grace period are dumped.  The new
trigger_single_cpu_backtrace() is used to make this happen, as
suggested by Josh Poimboeuf.

Reported-by: Vince Weaver <>
Signed-off-by: Paul E. McKenney <>
Cc: Petr Mladek <>
Cc: Peter Zijlstra <>
Reviewed-by: Josh Poimboeuf <>
Reviewed-by: Petr Mladek <>
Reviewed-by: Josh Triplett <>
 kernel/rcu/tree.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5a4aaad75e76..d7b63b88434b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1278,7 +1278,10 @@ static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
- * Dump stacks of all tasks running on stalled CPUs.
+ * Dump stacks of all tasks running on stalled CPUs.  First try using
+ * NMIs, but fall back to manual remote stack tracing on architectures
+ * that don't support NMI-based stack dumps.  The NMI-triggered stack
+ * traces are more accurate because they are printed by the target CPU.
 static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
@@ -1288,11 +1291,10 @@ static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
 	rcu_for_each_leaf_node(rsp, rnp) {
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
-		if (rnp->qsmask != 0) {
-			for_each_leaf_node_possible_cpu(rnp, cpu)
-				if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
+		for_each_leaf_node_possible_cpu(rnp, cpu)
+			if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
+				if (!trigger_single_cpu_backtrace(cpu))
-		}
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

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