On Wed, Jul 19, 2023 at 10:50:50PM +0200, Peter Zijlstra wrote: > > The below cures things; Josh, did I miss anything? > > > > --- > > arch/x86/entry/entry_64.S | 14 +++++++++++++- > > 1 file changed, 13 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S > > index 91f6818884fa..cfe7882ea9ae 100644 > > --- a/arch/x86/entry/entry_64.S > > +++ b/arch/x86/entry/entry_64.S > > @@ -285,7 +285,14 @@ SYM_FUNC_END(__switch_to_asm) > > */ > > .pushsection .text, "ax" > > SYM_CODE_START(ret_from_fork_asm) > > - UNWIND_HINT_REGS > > + /* > > + * This is the start of the kernel stack; even through there's a regs > > + * set at the top, there is no real exception frame and one cannot > > + * unwind further. This is the end. > > + * > > + * This ensures stack unwinds of kernel threads hit a known good state. > > + */ > > + UNWIND_HINT_END_OF_STACK The comments may be a bit superfluous (to me at least) but the patch looks fine. > So unwind_orc.c:unwind_next_frame() will terminate on this hint *or* on > user_mode(state->regs). > > AFAICT way things are set up in copy_thread(), user_mode() will not be > true -- after all there is no usermode, the kthread would first have to > exec() something to create a usermode. > > Yet I'm wondering if perhaps we should spoof the regs to make > user_mode() true and auto-terminate without this explicit hint. I'm not sure that would be worth the trouble / cleverness. The hint is straightforward IMO. > Josh, do you remember the rationale for all this? For what exactly :-) -- Josh
