[tip: core/urgent] rcu: Don't acquire lock in NMI handler in rcu_nmi_enter_common()

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/urgent branch of tip:

Commit-ID:     bf37da98c51825c90432d340e135cced37a7460d
Gitweb:        https://git.kernel.org/tip/bf37da98c51825c90432d340e135cced37a7460d
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Thu, 12 Mar 2020 16:55:07 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Sun, 05 Apr 2020 14:22:15 -07:00

rcu: Don't acquire lock in NMI handler in rcu_nmi_enter_common()

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@xxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 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 550193a..2c17859 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -825,7 +825,7 @@ 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)) {