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Date: Thu, 19 Oct 2017 11:14:38 +0900 From: Joonsoo Kim <iamjoonsoo.kim@....com> To: Thomas Gleixner <tglx@...utronix.de> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>, kernel test robot <xiaolong.ye@...el.com>, Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>, Denys Vlasenko <dvlasenk@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Jiri Slaby <jslaby@...e.cz>, Mike Galbraith <efault@....de>, Peter Zijlstra <peterz@...radead.org>, LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>, linux-mm <linux-mm@...ck.org>, Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter <cl@...ux.com> Subject: Re: [lkp-robot] [x86/kconfig] 81d3871900: BUG:unable_to_handle_kernel On Wed, Oct 18, 2017 at 03:15:03PM +0200, Thomas Gleixner wrote: > On Wed, 18 Oct 2017, Linus Torvalds wrote: > > On Tue, Oct 17, 2017 at 3:33 AM, Joonsoo Kim <iamjoonsoo.kim@....com> wrote: > > > > > > It looks like a compiler bug. The code of slob_units() try to read two > > > bytes at ffff88001c4afffe. It's valid. But the compiler generates > > > wrong code that try to read four bytes. > > > > > > static slobidx_t slob_units(slob_t *s) > > > { > > > if (s->units > 0) > > > return s->units; > > > return 1; > > > } > > > > > > s->units is defined as two bytes in this setup. > > > > > > Wrongly generated code for this part. > > > > > > 'mov 0x0(%rbp), %ebp' > > > > > > %ebp is four bytes. > > > > > > I guess that this wrong four bytes read cross over the valid memory > > > boundary and this issue happend. > > > > Hmm. I can see why the compiler would do that (16-bit accesses are > > slow), but it's definitely wrong. > > > > Does it work ok if that slob_units() code is written as > > > > static slobidx_t slob_units(slob_t *s) > > { > > int units = READ_ONCE(s->units); > > > > if (units > 0) > > return units; > > return 1; > > } > > > > which might be an acceptable workaround for now? > > Discussed exactly that with Peter Zijlstra yesterday, but we came to the > conclusion that this is a whack a mole game. It might fix this slob issue, > but what guarantees that we don't have the same problem in some other > place? Just duct taping this particular instance makes me nervous. I have checked that above patch works fine but I agree with Thomas. > Joonsoo says: > > > gcc 4.8 and 4.9 fails to generate proper code. gcc 5.1 and > > the latest version works fine. > > > I guess that this problem is related to the corner case of some > > optimization feature since minor code change makes the result > > different. And, with -O2, proper code is generated even if gcc 4.8 is > > used. > > So it would be useful to figure out which optimization bit is causing that > and blacklist it for the affected compiler versions. I have tried it but cannot find any clue. What I did is that compiling with -O2 and disabling some options to make option list as same as -Os. Some guide line is roughly mentioned in gcc man page. However, I cannot reproduce the issue by this way. Thanks.
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