Re: RCU stall leading to deadlock warning

[Paul E. McKenney]

On Wed, Dec 16, 2020 at 09:54:42AM -0800, Paul E. McKenney wrote:
> On Wed, Dec 16, 2020 at 05:29:39PM +0000, Qais Yousef wrote:
> > Hi Paul
> > 
> > We hit the below splat a couple of days ago in our testing. Sadly I can't
> > reproduce it. And it was on android-mainline branch..
> > 
> > It's the deadlock message that bothers me. I can't see how we could have ended
> > there. We detect a stall and when trying to dump the stack LOCKDEP spits the
> > warning.
> > 
> > Maybe should take this report with a pinch of salt since it wasn't on mainline.
> > I just thought it might be something worth sharing in case you can actually
> > spot something obvious that I can't see. If I got more info or a reproducer
> > I will share them.
> > 
> > The failure was triggered twice on that day running 2 different tests.
> 
> This looks like the same problem that Mark Rutland's recent patch series
> was fixing.  Do you have this series applied?
> 
> lore.kernel.org/lkml/20201126123602.23454-1-mark.rutland@xxxxxxx

I would not expect the patch below to help given what your RCU CPU stall
warning looks like, but just in case...

(Full disclosure: Peter fixed a bug of mine, filenames notwithstanding.)

							Thanx, Paul

------------------------------------------------------------------------

commit f355d19f94bf4361d641fb3dbb9ece0fbac766f8
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date:   Sat Aug 29 10:22:24 2020 -0700

    sched/core: Allow try_invoke_on_locked_down_task() with irqs disabled
    
    The try_invoke_on_locked_down_task() function currently requires
    that interrupts be enabled, but it is called with interrupts
    disabled from rcu_print_task_stall(), resulting in an "IRQs not
    enabled as expected" diagnostic.  This commit therefore updates
    try_invoke_on_locked_down_task() to use raw_spin_lock_irqsave() instead
    of raw_spin_lock_irq(), thus allowing use from either context.
    
    Link: https://lore.kernel.org/lkml/000000000000903d5805ab908fc4@xxxxxxxxxx/
    Link: https://lore.kernel.org/lkml/20200928075729.GC2611@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
    Reported-by: syzbot+cb3b69ae80afd6535b0e@xxxxxxxxxxxxxxxxxxxxxxxxx
    Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b2d6898..4abf041 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2989,7 +2989,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 
 /**
  * try_invoke_on_locked_down_task - Invoke a function on task in fixed state
- * @p: Process for which the function is to be invoked.
+ * @p: Process for which the function is to be invoked, can be @current.
  * @func: Function to invoke.
  * @arg: Argument to function.
  *
@@ -3007,12 +3007,11 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
  */
 bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct task_struct *t, void *arg), void *arg)
 {
-	bool ret = false;
 	struct rq_flags rf;
+	bool ret = false;
 	struct rq *rq;
 
-	lockdep_assert_irqs_enabled();
-	raw_spin_lock_irq(&p->pi_lock);
+	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
 	if (p->on_rq) {
 		rq = __task_rq_lock(p, &rf);
 		if (task_rq(p) == rq)
@@ -3029,7 +3028,7 @@ bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct t
 				ret = func(p, arg);
 		}
 	}
-	raw_spin_unlock_irq(&p->pi_lock);
+	raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);
 	return ret;
 }