Re: [PATCH RFC tip/core/rcu 1/2] rcu: Don't acquire lock in NMI handler in rcu_nmi_enter_common()

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Mar 12, 2020 at 11:52:01PM -0400, Joel Fernandes wrote:
> On Thu, Mar 12, 2020 at 07:40:45PM -0700, paulmck@kernel.org wrote:
> > 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.
> 
> 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@joelfernandes.org>

Applied, thank you!!!

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> 
> > Reported-by: Thomas Gleixner <tglx@linutronix.de>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.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 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
> >