Re: [PATCH] bpf: force inc_active()/dec_active() to be inline functions

[Alexei Starovoitov]

On Sun, Jul 23, 2023 at 11:32 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> >> If so, why can't we improve the compiler ?
> >
> > Agree.
> > Sounds like a compiler bug.
>
> I don't know what you might want to change in the compiler
> to avoid this. Compilers are free to decide which functions to
> inline in the absence of noinline or always_inline flags.

Clearly a compiler bug.
Compilers should not produce false positive warnings regardless
how inlining went and optimizations performed.


> One difference between gcc and clang is that gcc tries to
> be smart about warnings by using information from inlining
> to produce better warnings, while clang never uses information
> across function boundaries for generated warnings, so it won't
> find this one, but also would ignore an unconditional use
> of the uninitialized variable.
>
> >> If we have to change the kernel, what about the change below?
> >
> > To workaround the compiler bug we can simply init flag=0 to silence
> > the warn, but even that is silly. Passing flag=0 into irqrestore is buggy.
>
> Maybe inc_active() could return the flags instead of modifying
> the stack variable? that would also result in slightly better
> code when it's not inlined.

Which gcc are we talking about here that is so buggy?