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Subject[PATCH 5.6 64/71] rcu: Dont acquire lock in NMI handler in rcu_nmi_enter_common()
Date
From: Paul E. McKenney <paulmck@kernel.org>

commit bf37da98c51825c90432d340e135cced37a7460d upstream.

The rcu_nmi_enter_common() function can be invoked both in interrupt
and NMI handlers. If it is invoked from process context (as opposed
to userspace or idle context) on a nohz_full CPU, it might acquire the
CPU's leaf rcu_node structure's ->lock. Because this lock is held only
with interrupts disabled, this is safe from an interrupt handler, but
doing so from an NMI handler can result in self-deadlock.

This commit therefore adds "irq" to the "if" condition so as to only
acquire the ->lock from irq handlers or process context, never from
an NMI handler.

Fixes: 5b14557b073c ("rcu: Avoid tick_dep_set_cpu() misordering")
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: <stable@vger.kernel.org> # 5.5.x
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/rcu/tree.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -816,7 +816,7 @@ static __always_inline void rcu_nmi_ente
rcu_cleanup_after_idle();

incby = 1;
- } else if (tick_nohz_full_cpu(rdp->cpu) &&
+ } else if (irq && tick_nohz_full_cpu(rdp->cpu) &&
rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
READ_ONCE(rdp->rcu_urgent_qs) && !rdp->rcu_forced_tick) {
raw_spin_lock_rcu_node(rdp->mynode);

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