On Thu, Mar 12, 2020 at 07:40:45PM -0700, paulmck@xxxxxxxxxx wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
>
> 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.
I think Peter's new lockdep changes for NMI would also catch this issue.
>
> Fixes: 5b14557b073c ("rcu: Avoid tick_dep_set_cpu() misordering")
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
thanks,
- Joel
> Reported-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 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 d3f52c3..f7d3e48 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)) {
> --
> 2.9.5
>
