Messages in this thread | | | Subject | Re: [PATCH] jump_label: align jump_entry table to at least 4-bytes | From | Jason Baron <> | Date | Mon, 27 Feb 2017 14:18:54 -0500 |
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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
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