Re: [PATCH v2 4/4] dynamic_debug: add jump label support

[Chris Metcalf]

On 7/5/2016 4:57 PM, Jason Baron wrote:
> On 07/01/2016 03:30 PM, Chris Metcalf wrote:
> > On 6/10/2016 5:28 PM, Jason Baron wrote:
> > > Hi Arnd,
> > >
> > > Ok, I managed to reproduce this on tile and sparc64 by adding
> > > static_branch_[un]likely() to __exit functions as you mentioned.
> > > Although I didn't find the actual broken config.
> > >
> > > I think its only an issue on those 2 arches b/c they have jump
> > > label support and discard __exit text at build time (most
> > > arches seem to do it at run-time). Thus, we can end up with
> > > references in the __jump_table to addresses that may be in an
> > > __exit section. The jump label code already protects itself
> > > from touch code in the init sections after it has been freed.
> > > Thus, simply having functions marked with __exit in the init
> > > section is sufficient here.
> > How does this patch work instead?
> >
> > diff --git a/arch/tile/kernel/vmlinux.lds.S
> > b/arch/tile/kernel/vmlinux.lds.S
> > index 378f5d8d1ec8..5e83d2689def 100644
> > --- a/arch/tile/kernel/vmlinux.lds.S
> > +++ b/arch/tile/kernel/vmlinux.lds.S
> > @@ -58,7 +58,15 @@ SECTIONS
> >     _etext = .;
> >
> >     /* "Init" is divided into two areas with very different virtual
> > addresses. */
> > -  INIT_TEXT_SECTION(PAGE_SIZE)
> > +  . = ALIGN(PAGE_SIZE);
> > +  .init.text : AT(ADDR(.init.text) - LOAD_OFFSET) {
> > +    VMLINUX_SYMBOL(_sinittext) = .;
> > +    INIT_TEXT
> > +#ifdef CONFIG_JUMP_LABEL  /* __jump_table may reference __exit text */
> > +    EXIT_TEXT
> > +#endif
> > +    VMLINUX_SYMBOL(_einittext) = .;
> > +  }
> >
> >     /* Now we skip back to PAGE_OFFSET for the data. */
> >     . = (. - TEXT_OFFSET + PAGE_OFFSET);
>
> Hi Chris,
>
> Thanks for the patch. I can confirm that it resolves the following
> compilation issue:
>
> `.exit.text' referenced in section `__jump_table' of fs/built-in.o:
> defined in discarded section `.exit.text' of fs/built-in.o
> `.exit.text' referenced in section `__jump_table' of fs/built-in.o:
> defined in discarded section `.exit.text' of fs/built-in.o
>
> I was able to create the issue by simply adding a
> static_key_unlikely() branch to an __exit section that ends
> up being built-in (non-module).
>
> So this issue is really independent of this patch series...
> Should I add your patch to my series when I re-post?

I'm happy to just take a version of this into the tile tree instead, since
you're right, it's a pre-existing problem.  I'll post the revised change
shortly and cc you.  Thanks!

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com