On 02/27/2017 01:57 PM, David Daney wrote:

On 02/27/2017 10:49 AM, Jason Baron wrote:

quoted

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.

[...]

quoted

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