Messages in this thread | | | Subject | Re: [PATCH] jump_label: align jump_entry table to at least 4-bytes | From | David Daney <> | Date | Mon, 27 Feb 2017 11:59:50 -0800 |
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On 02/27/2017 11:18 AM, Jason Baron wrote: > > > 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...
Yes, I just looked at that. The ALIGN(8), combined with the fact that on MIPS, WORD_INSN will always be of size 4 or 8 (32-bit vs. 64-bit kernel), seems to make any additional .balign completely unnecessary.
Really, I think you need to figure out why on powerpc the alignments are getting screwed up. The struct jump_entry entries are on a stride of 12 (for a 32-bit kernel) or 24 (for a 64-bit kernel), any alignment padding added by .balign 4 will surely screw that up.
This patch seems like it is papering over a bigger issue that is not understood.
I think a proper fix would be to fix whatever is causing the powerpc JUMP_ENTRY_TYPE to misbehave.
> > 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.
For me the size is not the important issue, it is the alignment of the struct jump_entry entries in the table. I don't understand how your patch helps, and I cannot Acked-by unless I understand what is being done and can see that it is both correct and necessary.
David.
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