lkml.org 
[lkml]   [2017]   [Feb]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] jump_label: align jump_entry table to at least 4-bytes
From
Date


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

\
 
 \ /
  Last update: 2017-02-27 20:21    [W:0.057 / U:0.060 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site