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

[Mark Rutland]

On Fri, Nov 20, 2020 at 06:39:28AM -0800, Paul E. McKenney wrote:
> On Fri, Nov 20, 2020 at 03:19:28PM +0100, Marco Elver wrote:
> > 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?

I'm currently looking at it. I had been trying to shift things to C for
a while, and right now I'm trying to fix the lockdep state tracking,
which is requiring untangling lockdep/rcu/tracing.

The main issue I see remaining atm is that we don't save/restore the
lockdep state over exceptions taken from kernel to kernel. That could
result in lockdep thinking IRQs are disabled when they're actually
enabled (because code in the nested context might do a save/restore
while IRQs are disabled, then return to a context where IRQs are
enabled), but AFAICT shouldn't result in the inverse in most cases since
the non-NMI handlers all call lockdep_hardirqs_disabled().

I'm at a loss to explaim the rcu vs ftrace bits, so if you have any
pointers to the issuies ween with the x86 rework that'd be quite handy.

Thanks,
Mark.

> 
> 							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