On Thu, Mar 04, 2021 at 04:59PM +0000, Mark Rutland wrote: > On Thu, Mar 04, 2021 at 04:30:34PM +0100, Marco Elver wrote: > > On Thu, 4 Mar 2021 at 15:57, Mark Rutland <mark.rutl...@arm.com> wrote: > > > [adding Mark Brown] > > > > > > The bigger problem here is that skipping is dodgy to begin with, and > > > this is still liable to break in some cases. One big concern is that > > > (especially with LTO) we cannot guarantee the compiler will not inline > > > or outline functions, causing the skipp value to be too large or too > > > small. That's liable to happen to callers, and in theory (though > > > unlikely in practice), portions of arch_stack_walk() or > > > stack_trace_save() could get outlined too. > > > > > > Unless we can get some strong guarantees from compiler folk such that we > > > can guarantee a specific function acts boundary for unwinding (and > > > doesn't itself get split, etc), the only reliable way I can think to > > > solve this requires an assembly trampoline. Whatever we do is liable to > > > need some invasive rework. > > > > Will LTO and friends respect 'noinline'? > > I hope so (and suspect we'd have more problems otherwise), but I don't > know whether they actually so. > > I suspect even with 'noinline' the compiler is permitted to outline > portions of a function if it wanted to (and IIUC it could still make > specialized copies in the absence of 'noclone'). > > > One thing I also noticed is that tail calls would also cause the stack > > trace to appear somewhat incomplete (for some of my tests I've > > disabled tail call optimizations). > > I assume you mean for a chain A->B->C where B tail-calls C, you get a > trace A->C? ... or is A going missing too?
Correct, it's just the A->C outcome. > > Is there a way to also mark a function non-tail-callable? > > I think this can be bodged using __attribute__((optimize("$OPTIONS"))) > on a caller to inhibit TCO (though IIRC GCC doesn't reliably support > function-local optimization options), but I don't expect there's any way > to mark a callee as not being tail-callable. I don't think this is reliable. It'd be __attribute__((optimize("-fno-optimize-sibling-calls"))), but doesn't work if applied to the function we do not want to tail-call-optimize, but would have to be applied to the function that does the tail-calling. So it's a bit backwards, even if it worked. > Accoding to the GCC documentation, GCC won't TCO noreturn functions, but > obviously that's not something we can use generally. > > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes Perhaps we can ask the toolchain folks to help add such an attribute. Or maybe the feature already exists somewhere, but hidden. +Cc linux-toolcha...@vger.kernel.org > > But I'm also not sure if with all that we'd be guaranteed the code we > > want, even though in practice it might. > > True! I'd just like to be on the least dodgy ground we can be. It's been dodgy for a while, and I'd welcome any low-cost fixes to make it less dodgy in the short-term at least. :-) Thanks, -- Marco