linux-next: stall warnings and deadlock on Arm64 (was: [PATCH] kfence: Avoid stalling...)

[Will Deacon]

On Thu, Nov 19, 2020 at 01:35:12PM -0800, Paul E. McKenney wrote:
> On Thu, Nov 19, 2020 at 08:38:19PM +0100, Marco Elver wrote:
> > On Thu, Nov 19, 2020 at 10:48AM -0800, Paul E. McKenney wrote:
> > > On Thu, Nov 19, 2020 at 06:02:59PM +0100, Marco Elver wrote:
> 
> [ . . . ]
> 
> > > > I can try bisection again, or reverting some commits that might be
> > > > suspicious? But we'd need some selection of suspicious commits.
> > > 
> > > The report claims that one of the rcu_node ->lock fields is held
> > > with interrupts enabled, which would indeed be bad.  Except that all
> > > of the stack traces that it shows have these locks held within the
> > > scheduling-clock interrupt handler.  Now with the "rcu: Don't invoke
> > > try_invoke_on_locked_down_task() with irqs disabled" but without the
> > > "sched/core: Allow try_invoke_on_locked_down_task() with irqs disabled"
> > > commit, I understand why.  With both, I don't see how this happens.
> > 
> > I'm at a loss, but happy to keep bisecting and trying patches. I'm also
> > considering:
> > 
> > 	Is it the compiler? Probably not, I tried 2 versions of GCC.
> > 
> > 	Can we trust lockdep to precisely know IRQ state? I know there's
> > 	been some recent work around this, but hopefully we're not
> > 	affected here?
> > 
> > 	Is QEMU buggy?
> > 
> > > At this point, I am reduced to adding lockdep_assert_irqs_disabled()
> > > calls at various points in that code, as shown in the patch below.
> > > 
> > > At this point, I would guess that your first priority would be the
> > > initial bug rather than this following issue, but you never know, this
> > > might well help diagnose the initial bug.
> > 
> > I don't mind either way. I'm worried deadlocking the whole system might
> > be worse.
> 
> Here is another set of lockdep_assert_irqs_disabled() calls on the
> off-chance that they actually find something.

FWIW, arm64 is known broken wrt lockdep and irq tracing atm. Mark has been
looking at that and I think he is close to having something workable.

Mark -- is there anything Marco and Paul can try out?

Will