On 06/10/2016 05:54 AM, Arnd Bergmann wrote:
> On Friday, May 20, 2016 5:16:36 PM CEST Jason Baron wrote:
>> Although dynamic debug is often only used for debug builds, sometimes its
>> enabled for production builds as well. Minimize its impact by using jump
>> labels. This reduces the text section by 7000+ bytes in the kernel image
>> below. It does increase data, but this should only be referenced when
>> changing the direction of the branches, and hence usually not in cache.
>>
>> text data bss dec hex filename
>> 8194852 4879776 925696 14000324 d5a0c4 vmlinux.pre
>> 8187337 4960224 925696 14073257 d6bda9 vmlinux.post
>>
>> Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
>> ---
>
> This causes problems for some of my randconfig builds, when a dynamic
> debug call is used inside of an __exit function:
>
> `.exit.text' referenced in section `__jump_table' of drivers/built-in.o: defined in discarded section `.exit.text' of drivers/built-in.o
> `.exit.text' referenced in section `__jump_table' of drivers/built-in.o: defined in discarded section `.exit.text' of drivers/built-in.o
>
> Arnd
>
Hi Arnd,
Ok, I managed to reproduce this on tile and sparc64 by adding
static_branch_[un]likely() to __exit functions as you mentioned.
Although I didn't find the actual broken config.
I think its only an issue on those 2 arches b/c they have jump
label support and discard __exit text at build time (most
arches seem to do it at run-time). Thus, we can end up with
references in the __jump_table to addresses that may be in an
__exit section. The jump label code already protects itself
from touch code in the init sections after it has been freed.
Thus, simply having functions marked with __exit in the init
section is sufficient here.
I tried the following patches on tile and sparc and they
at least now compile. If anybody has real h/w that would
be helpful :)
Thanks,
-Jason
--- a/arch/sparc/kernel/vmlinux.lds.S
+++ b/arch/sparc/kernel/vmlinux.lds.S
@@ -151,6 +151,12 @@ SECTIONS
PERCPU_SECTION(SMP_CACHE_BYTES)
. = ALIGN(PAGE_SIZE);
+ .exit.text : {
+ EXIT_TEXT
+ }
+
+ . = ALIGN(PAGE_SIZE);
+
__init_end = .;
BSS_SECTION(0, 0, 0)
_end = . ;
--- a/arch/tile/kernel/vmlinux.lds.S
+++ b/arch/tile/kernel/vmlinux.lds.S
@@ -58,7 +58,21 @@ SECTIONS
_etext = .;
/* "Init" is divided into two areas with very different virtual
addresses. */
+ . = ALIGN(PAGE_SIZE);
+ .init.begin : AT(ADDR(.init.begin) - LOAD_OFFSET) {
+ __init_begin = .; /* paired with __init_end */
+ }
+
INIT_TEXT_SECTION(PAGE_SIZE)
+ .exit.text : AT(ADDR(.exit.text) - LOAD_OFFSET) {
+ EXIT_TEXT
+ }
+
+ . = ALIGN(PAGE_SIZE);
+ /* freed after init ends here */
+ .init.end : AT(ADDR(.init.end) - LOAD_OFFSET) {
+ __init_end = .;
+ }
/* Now we skip back to PAGE_OFFSET for the data. */
. = (. - TEXT_OFFSET + PAGE_OFFSET);
--
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