From: Marco Elver <elver@google.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
LKML <linux-kernel@vger.kernel.org>,
linuxppc-dev@lists.ozlabs.org,
kasan-dev <kasan-dev@googlegroups.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
broonie@kernel.org, linux-toolchains@vger.kernel.org
Subject: Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends
Date: Thu, 4 Mar 2021 18:25:33 +0100 [thread overview]
Message-ID: <YEEYDSJeLPvqRAHZ@elver.google.com> (raw)
In-Reply-To: <20210304165923.GA60457@C02TD0UTHF1T.local>
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.rutland@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-toolchains@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
next prev parent reply other threads:[~2021-03-04 17:27 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-03 14:09 [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends Christophe Leroy
2021-03-03 14:38 ` Marco Elver
2021-03-03 14:52 ` Christophe Leroy
2021-03-03 15:20 ` Marco Elver
2021-03-04 14:57 ` Mark Rutland
2021-03-04 15:30 ` Marco Elver
2021-03-04 16:59 ` Mark Rutland
2021-03-04 17:25 ` Marco Elver [this message]
2021-03-04 17:54 ` Nick Desaulniers
2021-03-04 19:24 ` Segher Boessenkool
2021-03-05 6:38 ` Christophe Leroy
2021-03-05 18:16 ` Segher Boessenkool
2021-03-04 18:01 ` Mark Rutland
2021-03-04 18:22 ` Marco Elver
2021-03-04 18:51 ` Mark Rutland
2021-03-04 19:01 ` Marco Elver
2021-03-05 12:04 ` Mark Rutland
2021-03-04 21:54 ` Segher Boessenkool
2021-03-09 16:05 ` Mark Rutland
2021-03-09 22:05 ` Segher Boessenkool
2021-03-10 11:32 ` Mark Rutland
2021-03-10 17:37 ` Segher Boessenkool
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