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Thu, 04 Mar 2021 09:25:39 -0800 (PST) Received: from elver.google.com ([2a00:79e0:15:13:adef:40fb:49ed:5ab6]) by smtp.gmail.com with ESMTPSA id f17sm36439007wru.31.2021.03.04.09.25.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 04 Mar 2021 09:25:38 -0800 (PST) Date: Thu, 4 Mar 2021 18:25:33 +0100 From: Marco Elver To: Mark Rutland Subject: Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends Message-ID: References: <1802be3e-dc1a-52e0-1754-a40f0ea39658@csgroup.eu> <20210304145730.GC54534@C02TD0UTHF1T.local> <20210304165923.GA60457@C02TD0UTHF1T.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210304165923.GA60457@C02TD0UTHF1T.local> User-Agent: Mutt/2.0.5 (2021-01-21) X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Catalin Marinas , Will Deacon , LKML , broonie@kernel.org, Paul Mackerras , kasan-dev , linux-toolchains@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Linux ARM Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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 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