Re: [PATCH 3/4] x86: Rewrite switch_to() code

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Mon, May 23, 2016 at 06:49:03AM -0500, Josh Poimboeuf wrote:
> On Mon, May 23, 2016 at 06:47:22AM -0500, Josh Poimboeuf wrote:
> > On Mon, May 23, 2016 at 07:14:14AM -0400, Brian Gerst wrote:
> > > On Sun, May 22, 2016 at 10:34 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > > > On Sun, May 22, 2016 at 10:59:38AM -0700, Andy Lutomirski wrote:
> > > >> cc: Josh Poimboeuf: do you care about the exact stack layout of the
> > > >> bottom of the stack of an inactive task?
> > > >
> > > > So there's one minor issue with this patch, relating to unwinding the
> > > > stack of a newly forked task.  For detecting reliable stacks, the
> > > > unwinder needs to unwind all the way to the syscall pt_regs to make sure
> > > > the stack is sane.  But for newly forked tasks, that won't be possible
> > > > here because the unwinding will stop at the fork_frame instead.
> > > >
> > > > So from an unwinder standpoint it might be nice for copy_thread_tls() to
> > > > place a frame pointer on the stack next to the ret_from_fork return
> > > > address, so that it would resemble an actual stack frame.  The frame
> > > > pointer could probably just be hard-coded to zero.  And then the first
> > > > bp in fork_frame would need to be a pointer to it instead of zero.  That
> > > > would make it nicely resemble the stack of any other task.
> > > >
> > > > Alternatively I could teach the unwinder that if the unwinding starts at
> > > > the fork_frame offset from the end of the stack page, and the saved rbp
> > > > is zero, it can assume that it's a newly forked task.  But that seems a
> > > > little more brittle to me, as it requires the unwinder to understand
> > > > more of the internal workings of the fork code.
> > > >
> > > > But overall I think this patch is a really nice cleanup, and other than
> > > > the above minor issue it should be fine with my reliable unwinder, since
> > > > rbp is still at the top of the stack.
> > > 
> > > Ok, how about if it pushed RBP first, then we teach get_wchan() to add
> > > the fixed offset from thread.sp to get bp?  that way it don't have to
> > > push it twice.
> > 
> > In theory I like the idea, and it would work: the unwinder could just
> > use the inactive_task_frame struct (as Andy suggested) to find the frame
> > pointer.
> > 
> > But I suspect it would break all existing unwinders, both in-tree and
> > out-of-tree.  The only out-of-tree one I know of is crash, not sure if
> > there are more out there.
> 
> I should mention it would only affect those unwinders which know how to
> do sleeping kernel tasks.  So generic tools like gdb wouldn't be
> affected.

[continuing my conversation with myself...]

To clarify, I still think we should do it.  The stack format of a
sleeping task isn't exactly an ABI, and I wouldn't expect many tools to
rely on it.  I can help with the fixing of in-tree unwinders if needed.

Or I could even do the moving of the frame pointer as a separate patch
on top of this one, since it might cause breakage elsewhere.

Adding Dave Anderson (crash maintainer) to cc, as an FYI.

-- 
Josh