Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

On Thu, Oct 31, 2024 at 02:22:48PM -0700, Andrii Nakryiko wrote:
> > Problem is, the unwinder doesn't know in advance which tasks will be
> > unwound.
> >
> > Its first clue is unwind_user_register(), would it make sense for the
> > caller to clarify whether all tasks need to be unwound or only a
> > specific subset?
> >
> > Its second clue is unwind_user_deferred(), which is called for the task
> > itself.  But by then it's too late because it needs to access the
> > per-task data from (potentially) irq context so it can't do a lazy
> > allocation.
> >
> > I'm definitely open to ideas...
> 
> The laziest thing would be to perform GFP_ATOMIC allocation, and if
> that fails, oops, too bad, no stack trace for you (but, generally
> speaking, no big deal). Advantages are clear, though, right? Single
> pointer in task_struct, which most of the time will be NULL, so no
> unnecessary overheads.

GFP_ATOMIC is limited, I don't think we want the unwinder to trigger
OOM.

> It's the last point that's important to make usability so much
> simpler, avoiding unnecessary custom timeouts and stuff like that.
> Regardless whether stack trace capture is success or not, user is
> guaranteed to get a "notification" about the outcome.
> 
> Hope this helps.
> 
> But basically, if I I called unwind_user_deferred(), I expect to get
> some callback, guaranteed, with the result or failure. The only thing
> that's not guaranteed (and which makes timeouts bad) is *when* this
> will happen. Because stack trace capture can be arbitrarily delayed
> and stuff. That's fine, but that also shows why timeout is tricky and
> necessarily fragile.

That sounds reasonable.  In the OOM error case I can just pass a small
(stack allocated) one-entry trace with only regs->ip.

-- 
Josh