Re: [RFC PATCH v1 12/16] unwind_user/backchain: Introduce back chain user space unwinding

[Heiko Carstens <hca@linux.ibm.com>]

On Thu, Jul 17, 2025 at 10:19:54PM -0700, Josh Poimboeuf wrote:
> On Thu, Jul 17, 2025 at 02:20:12PM +0200, Jens Remus wrote:
> > > Also, if distros aren't even compiling with -mbackchain, I wonder if we
> > > can just not do this altogether :-)
> > 
> > My original intent was to use unwind user's for_each_user_frame() to
> > replace the exiting stack tracing logic in arch_stack_walk_user_common()
> > in arch/s390/kernel/stacktrace.c, which currently supports backchain.
> > Given that for_each_user_frame() was made private in the latest unwind
> > user series version hinders me.  The use was also low, because the
> > currentl arch_stack_walk_user_common() implementation does not support
> > page faults, so that the attempt to use unwind user sframe would always
> > fail and fallback to unwind user backchain.  My hope was that somebody
> > with more Kernel skills could give me a few hints at how it could be
> > made to support deferred unwind. :-)
> 
> I believe stack_trace_save_user() is only used by ftrace, and that will
> no longer be needed once ftrace starts using unwind_user.
> 
> Maybe Heiko knows if that backchain user stacktrace code has any users?
> 
> If distros aren't building with -mbackchain, maybe backchain support can
> be considered obsoleted by sframe, and we can get away with not
> implementing it.

I guess that's a valid option. I know only of some special cases where
users compile everything on their own with -mbackchain to make this
work on a per-case basis. It shouldn't cause to much pain for them to
switch to sframe, as soon as that is available.