On Mon, May 23, 2016 at 10:03:54AM -0700, Andy Lutomirski wrote: > On Mon, May 23, 2016 at 9:46 AM, Josh Poimboeuf <[email protected]> wrote: > > 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 <[email protected]> > >> > > 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. > > Do you have any understanding of why there are so many unwinder > implementations? Your reliable unwinder seems to be yet another copy > of more or less the same code.
Yeah, there are way too many instantations of stacktrace_ops and there's definitely a lot of room for consolidation and simplification. There are different requirements needed by all the different codes relying on dump_trace(): - starting with a given pt_regs - starting with a given task - whether to skip sched code functions - whether to skip the random '?' ktext addresses found on the stack - whether frame pointers are enabled - whether the stack is reliable - output to an arch-independent "struct stack_trace" array So everybody implements their own callbacks for dump_trace(). It's kind of a big mess. > I'd like to see a single, high-quality unwinder implemented as a state > machine, along the lines of: > > struct unwind_state state; > unwind_start_inactive_task(&state, ...); or > unwind_start_pt_regs(&state, regs); or whatever. > unwind_next_frame(&state); > > where, after unwind_next_frame, state encodes whatever registers are > known (at least bp and ip, but all the GPRs would be nice and are > probably mandatory for DWARF) and an indication of whether this is a > real frame or a guessed frame (the things that currently show up as > '?'). I like the idea of a state machine. I'll probably end up doing something like that before introducing the DWARF unwinder. -- Josh
