lkml.org 
[lkml]   [2016]   [Jun]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patches in this message
    /
    SubjectRe: [PATCH v2 4/4] dynamic_debug: add jump label support
    From
    Date
    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@akamai.com>
    >> ---
    >
    > 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);
    \
     
     \ /
      Last update: 2016-06-11 00:01    [W:4.529 / U:1.788 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site