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

[Paul E. McKenney]

On Fri, Nov 20, 2020 at 03:19:28PM +0100, Marco Elver wrote:
> On Thu, Nov 19, 2020 at 01:35PM -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.
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit bcca5277df3f24db15e15ccc8b05ecf346d05169
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Date:   Thu Nov 19 13:30:33 2020 -0800
> > 
> >     rcu: Add lockdep_assert_irqs_disabled() to raw_spin_unlock_rcu_node() macros
> 
> None of those triggered either.
> 
> I found that disabling ftrace for some of kernel/rcu (see below) solved
> the stalls (and any mention of deadlocks as a side-effect I assume),
> resulting in successful boot.
> 
> Does that provide any additional clues? I tried to narrow it down to 1-2
> files, but that doesn't seem to work.

There were similar issues during the x86/entry work.  Are the ARM guys
doing arm64/entry work now?

							Thanx, Paul

> Thanks,
> -- Marco
> 
> ------ >8 ------
> 
> diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
> index 0cfb009a99b9..678b4b094f94 100644
> --- a/kernel/rcu/Makefile
> +++ b/kernel/rcu/Makefile
> @@ -3,6 +3,13 @@
>  # and is generally not a function of system call inputs.
>  KCOV_INSTRUMENT := n
>  
> +ifdef CONFIG_FUNCTION_TRACER
> +CFLAGS_REMOVE_update.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_sync.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_srcutree.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_tree.o = $(CC_FLAGS_FTRACE)
> +endif
> +
>  ifeq ($(CONFIG_KCSAN),y)
>  KBUILD_CFLAGS += -g -fno-omit-frame-pointer
>  endif