[PATCH 2/2] ARM: unwind: enable dumping stacks for SMP && ARM_UNWIND

[Dave Martin]

On Tue, Oct 16, 2012 at 11:55:04AM +0100, Russell King - ARM Linux wrote:
> On Tue, Oct 16, 2012 at 11:12:01AM +0100, Dave Martin wrote:
> > On Mon, Oct 15, 2012 at 07:15:31PM -0700, Colin Cross wrote:
> > > About half the callers to unwind_frame end up limiting the number of
> > > frames they will follow before giving up, so I wasn't sure if I should
> > > put an arbitrary limit in unwind_frame or just make sure all callers
> > > are bounded.  Your idea of limiting same sp frames instead of total
> > > frames sounds better.  I can send a new patch that adds a new field to
> > > struct stackframe (which will need to be initialized everywhere, the
> > > struct is usually on the stack) and limits the recursion.  Any
> > > suggestion on the recursion limit?  I would never expect to see a real
> > > situation with more than a few, but on the other hand parsing the
> > > frames should be pretty fast so a high number (100?) shouldn't cause
> > > any user visible effect.
> > 
> > Talking to some tools guys about this, it sounds like there really
> > shouldn't be any stackless frame except for the leaf frame.  If there are
> > stackless functions they will probably not be visible in the frame chain
> > at all.  So it might make sense to have a pretty small limit.  Maybe it
> > could even be 1.  Cartainly a small number.
> > 
> > We should also add a check for whether the current and frame and previous
> > frame appear identical and abort if that's the case, if we don't do that
> > already.
> 
> The case that actually worries me is not the "end up looping for ever"
> case, but the effects of having the stack change while the unwinder is
> reading from it - for example:
> 
>                 /* pop R4-R15 according to mask */
>                 load_sp = mask & (1 << (13 - 4));
>                 while (mask) {
>                         if (mask & 1)
>                                 ctrl->vrs[reg] = *vsp++;
>                         mask >>= 1;
>                         reg++;
>                 }
> 
> Remember that for a running thread, the stack will be changing all the
> time while another CPU tries to read it to do the unwind, and also
> remember that the bottom of stack isn't really known.  All you have is
> the snapshot of the registers when the thread was last stopped by the
> scheduler, and that state probably isn't valid.

So long as the unwinder enforces continuous progress towards a fixed
limit, sooner or later the supposed bottom of the stack will be reached,
or the unwinder will encounter something which is recognised as garbage
and stop.

This the best we can hope for if trying to print a backtrace for a
thread without stopping it...  which admittedly seems quite a dodgy
thing to attempt.

Colin, what are the scenarios when we want to backtrace a thread while
it is actually running?

> So what you're asking is for the unwinder to produce a backtrace from
> a kernel stack which is possibly changing beneath it from an unknown
> current state.
> 
> This doesn't sound like a particularly bright thing to be doing...

Nonetheless, the changes are relevant to normal stack dumping too, since
when we take a fault the sp or stack may be corrupted, even if the
thread in question is stopped.  Being more robust against infinte loops
etc., still seems like a good idea ?

Cheers
---Dave