Messages in this thread | | | Date | Tue, 10 Sep 2019 14:24:15 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] arm64: fix unreachable code issue with cmpxchg |
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On Tue, Sep 10, 2019 at 10:04:24AM +0200, Arnd Bergmann wrote: > On Tue, Sep 10, 2019 at 9:46 AM Will Deacon <will@kernel.org> wrote: > > > > On Mon, Sep 09, 2019 at 10:21:35PM +0200, Arnd Bergmann wrote: > > > On arm64 build with clang, sometimes the __cmpxchg_mb is not inlined > > > when CONFIG_OPTIMIZE_INLINING is set. > > > > Hmm. Given that CONFIG_OPTIMIZE_INLINING has also been shown to break > > assignment of local 'register' variables on GCC, perhaps we should just > > disable that option for arm64 (at least) since we don't have any toolchains > > that seem to like it very much! I'd certainly prefer that over playing > > whack-a-mole with __always_inline. > > Right, but I can also see good reasons to keep going: > > - In theory, CONFIG_OPTIMIZE_INLINING is the right thing to do -- the compilers > also make some particularly bad decisions around inlining when each inline > turns into an __always_inline, as has been the case in Linux for a long time. > I think in most cases, we get better object code with CONFIG_OPTIMIZE_INLINING > and in the cases where this is worse, it may be better to fix the compiler. > The new "asm_inline" macro should also help with that.
Sure, in theory, but it looks like there isn't a single arm64 compiler out there which gets it right.
> - The x86 folks have apparently whacked most of the moles already, see this > commit from 2008 > > commit 3f9b5cc018566ad9562df0648395649aebdbc5e0 > Author: Ingo Molnar <mingo@elte.hu> > Date: Fri Jul 18 16:30:05 2008 +0200 > > x86: re-enable OPTIMIZE_INLINING > > re-enable OPTIMIZE_INLINING more widely. Jeff Dike fixed the remaining > outstanding issue in this commit: > > | commit 4f81c5350b44bcc501ab6f8a089b16d064b4d2f6 > | Author: Jeff Dike <jdike@addtoit.com> > | Date: Mon Jul 7 13:36:56 2008 -0400 > | > | [UML] fix gcc ICEs and unresolved externs > [...] > | This patch reintroduces unit-at-a-time for gcc >= 4.0, > bringing back the > | possibility of Uli's crash. If that happens, we'll debug it. > > it's still default-off and thus opt-in.
This appears to be fixing an ICE, whereas the issue reported recently for arm64 gcc was silent miscompilation of atomics in some cases. Unfortunately, I can't seem to find the thread :/ Mark, you were on that one too, right?
> - The inlining decisions of gcc and clang are already very different, and > the bugs we are finding around that are much more common than > the difference between CONFIG_OPTIMIZE_INLINING=y/n on a > given compiler.
Sorry, not sure that you're getting at here.
Anyway, the second version of your patch looks fine, but I would still prefer to go the extra mile and disable CONFIG_OPTIMIZE_INLINING altogether given that I don't think it's a safe option to enable for us.
Will
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