From mboxrd@z Thu Jan 1 00:00:00 1970 From: david@protonic.nl (David Jander) Date: Tue, 16 Sep 2014 13:42:09 +0200 Subject: gcc 4.8.3 miscompiles drivers/net/ethernet/freescale/fec_main.c ?! In-Reply-To: <20140910164920.3c448b40@archvile> References: <20140910110943.3673dbb4@archvile> <21520.17620.519242.597681@gargle.gargle.HOWL> <20140910164920.3c448b40@archvile> Message-ID: <20140916134209.7cc21f86@archvile> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Mikael, On Wed, 10 Sep 2014 16:49:20 +0200 David Jander wrote: > On Wed, 10 Sep 2014 14:32:20 +0200 > Mikael Pettersson wrote: > > > David Jander writes: > > > > > > Hi, > > > > > > I am seeing a strange problem when building a recent kernel with > > > gcc-4.8.3 for armv7-a that contains the following patch: > > > > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit?id=bfd4ecdd87d350e19457fe0d02fa1e046774c44e > > > > > > Unfortunately I am not good enough at reading ARM assembly output from > > > GCC to understand whats going wrong, so I am asking for help. > > > > > > I started noticing ethernet packet loss on a i.MX6 board after upgrading > > > the kernel from 3.16-rc-something to latest mainline. The problem is > > > very easy to reproduce so I started git-bisecting. Git bisect gave me > > > the above patch as the culprit, and indeed: Without the patch a > > > flood-ping goes fine (just one dot on screen, no lost packets). I apply > > > the patch and the dots start filling the screen instantly. > > > > > > I am compiling the kernel using Pengutronix's OSELAS toolchain version > > > 2013.12.1, which is based on linaro gcc-4.8.3 without any relevant > > > patches AFAIK. > > > > Linaro's toolchain is itself heavily modified compared to FSF gcc-4.8.3, > > so first please try a pure vanilla FSF gcc-4.8.3, and then a likewise > > vanilla gcc-4.9.1. If those also cause the malfunction, then you have > > proof for a bug in upstream gcc (or possibly undefined code in the kernel), > > otherwise the bug is likely Linaro's. > > Thanks. I will try to build gcc-4.8.3 from vanilla FSF sources and try to > reproduce the problem there. Do you think there is a chance this is still a > kernel bug? > I have assembly output of both working and broken cases (inlined and > non-inlined function). I can post them here or send to anyone who wants to > try to make sense of it.... This is getting weird: I have build both vanilla FSF toolchains: binutils-2.24 with gcc-4.8.3 and gcc-4.9.1 I can't quite make sense of the results, other than that there is a race-condition in the Linux kernel fec_main.c: gcc-4.8.3 with -O2: fails the same as with OSELAS.Toolchain/linaro gcc-4.8.3 gcc-4.8.3 with -Os: Seems to work correctly gcc-4.9.1 with -O2: Loses packets, but much less often than with gcc-4.8.3 -O2 gcc-4.9.1 with -Os: Fails even worse than with gcc-4.8.3 -O2 Any suggestion on how to make sense of this? For me this looks slightly more like a kernel bug than a compiler bug... > > > Compiling with -O2 breaks the code, while -Os seems to produce a > > > correctly working kernel. > > > > > > I decided to make changes to the code and see if I could find other ways > > > to "fix" the problem, and I got the following result: > > > > > > The above mentioned patch introduces the static function > > > fec_enet_hwtstamp() near line 1068 of fec_main.c. If I make an exact > > > copy of this function, where I only change the name (e.g. > > > fec_enet_hwtstamp2), and change one of the two places this function is > > > called to instead use the other name, GCC inlines both copies and the > > > problem disappears! > > > > > > Since I am not very good at GCC internals nor do I know this piece of > > > code in fec_main.c very well, I am asking here for help in hunting down > > > the real bug, which I suspect is in GCC... but I want to know for sure. > > > Best regards, -- David Jander Protonic Holland.