From: "Paul E. McKenney" paulmck@kernel.org

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

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 7dd7a17..573fd78 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -876,7 +876,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq) 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) && !READ_ONCE(rdp->rcu_forced_tick)) {