Re: [PATCH] compiler: enable CONFIG_OPTIMIZE_INLINING forcibly

[Masahiro Yamada]

Hi Will,

On Mon, Sep 30, 2019 at 9:18 PM Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Mon, Sep 30, 2019 at 09:05:11PM +0900, Masahiro Yamada wrote:
> > On Mon, Sep 30, 2019 at 8:26 PM Will Deacon <will@xxxxxxxxxx> wrote:
> > > On Fri, Sep 27, 2019 at 03:38:44PM -0700, Linus Torvalds wrote:
> > > > Soem of that code is pretty subtle. They have fixed register usage
> > > > (but the asm macros actually check them). And the inline asms clobber
> > > > the link register, but they do seem to clearly _state_ that they
> > > > clobber it, so who knows.
> > > >
> > > > Just based on the EFAULT, I'd _guess_ that it's some interaction with
> > > > the domain access control register (so that get/set_domain() thing).
> > > > But I'm not even sure that code is enabled for the Rpi2, so who
> > > > knows..
> > >
> > > FWIW, we've run into issues with CONFIG_OPTIMIZE_INLINING and local
> > > variables marked as 'register' where GCC would do crazy things and end
> > > up corrupting data, so I suspect the use of fixed registers in the arm
> > > uaccess functions is hitting something similar:
> > >
> > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91111
> >
> > No. Not similar at all.
>
> They're similar in that enabling CONFIG_OPTIMIZE_INLINING causes register
> variables to go wrong.

You are just looking at the result, not the cause.


> I agree that the ARM code looks dodgy with
> that call to uaccess_save_and_enable(), but there are __asmeq macros
> in there to try to catch that, so it's still very fishy.

I am totally fine with double-checking
the output from the compiler.


>
> > I fixed it already. See
> > https://lore.kernel.org/patchwork/patch/1132459/
>
> You fixed the specific case above for 32-bit ARM, but the arm64 case
> is due to a compiler bug. As it happens, we've reworked our atomics
> in 5.4 so that particular issue no longer triggers, but the fact remains
> that GCC has been shown to screw up explicit register allocation for
> perfectly legitimate code when giving the flexibility to move code out
> of line.
>
> > The problems are fixable by writing correct code.
>
> Right, in the compiler ;)

Not always.

You showed a compiler bug case for arm64.
The 32bit ARM is not the case.


>
> > I think we discussed this already.
>
> We did?


https://www.spinics.net/lists/arm-kernel/msg754567.html

This is a bug in the kernel code.



> > - There is nothing arch-specific in CONFIG_OPTIMIZE_INLINING
>
> Apart from the bugs... and even then, that's just based on reports.
>
> > - 'inline' is just a hint. It does not guarantee function inlining.
> >   This is standard.
> >
> > - The kernel macrofies 'inline' to add __attribute__((__always_inline__))
> >   This terrible hack must end.
>
> I'm all for getting rid of hacks, but not at the cost of correctness.
>
> > -  __attribute__((__always_inline__)) takes aways compiler's freedom,
> >    and prevents it from optimizing the code for -O2, -Os, or whatever.
>
> s/whatever/miscompiling the code/
>
> If it helps, here is more information about the arm64 failure which
> triggered the GCC bugzilla:
>
> https://www.spinics.net/lists/arm-kernel/msg730329.html
>
> Will

You put multiple references here and there,
but they are all about arch_atomic64_dec_if_positive().


--
Best Regards
Masahiro Yamada