Re: [PATCH] jump_label: align jump_entry table to at least 4-bytes

[Jason Baron]

On 02/27/2017 01:57 PM, David Daney wrote:
> On 02/27/2017 10:49 AM, Jason Baron wrote:
> > The core jump_label code makes use of the 2 lower bits of the
> > static_key::[type|entries|next] field. Thus, ensure that the jump_entry
> > table is at least 4-byte aligned.
> [...]
> > diff --git a/arch/mips/include/asm/jump_label.h
> > b/arch/mips/include/asm/jump_label.h
> > index e77672539e8e..243791f3ae71 100644
> > --- a/arch/mips/include/asm/jump_label.h
> > +++ b/arch/mips/include/asm/jump_label.h
> > @@ -31,6 +31,7 @@ static __always_inline bool
> > arch_static_branch(struct static_key *key, bool bran
> >      asm_volatile_goto("1:\t" NOP_INSN "\n\t"
> >          "nop\n\t"
> >          ".pushsection __jump_table,  \"aw\"\n\t"
> > +        ".balign 4\n\t"
> >          WORD_INSN " 1b, %l[l_yes], %0\n\t"
> >          ".popsection\n\t"
> >          : :  "i" (&((char *)key)[branch]) : : l_yes);
> > @@ -45,6 +46,7 @@ static __always_inline bool
> > arch_static_branch_jump(struct static_key *key, bool
> >      asm_volatile_goto("1:\tj %l[l_yes]\n\t"
> >          "nop\n\t"
> >          ".pushsection __jump_table,  \"aw\"\n\t"
> > +        ".balign 4\n\t"
> >          WORD_INSN " 1b, %l[l_yes], %0\n\t"
> >          ".popsection\n\t"
> >          : :  "i" (&((char *)key)[branch]) : : l_yes);
>
>
> I will speak only for the MIPS part.
>
> If the section is not already properly aligned, this change will add
> padding, which is probably not what we want.
>
> Have you ever seen a problem with misalignment in the real world?

Hi,

Yes, there was a WARN_ON() reported on POWER here:

https://lkml.org/lkml/2017/2/19/85

The WARN_ON() triggers if either of the 2 lsb are set. More 
specifically, its coming from 'static_key_set_entries()' from the 
following patch, which was recently added to linux-next:

https://lkml.org/lkml/2017/2/3/558

So this was not seen on mips, but I included all arches in this patch 
that I though might be affected.

> If so, I think a better approach might be to set properties on the
> __jump_table section to force the proper alignment, or do something in
> the linker script.

So in include/asm-generic/vmlinux.lds.h, we are already setting 
'ALIGN(8)', but that does not appear to be sufficient for POWER...

Also, I checked the size of the vmlinux generated after this change on 
all 4 arches, and it was rather minimal. I think POWER increased the 
most, but the other arches increased by only a few bytes.

Thanks,

-Jason