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    SubjectRe: [PATCH 0/2] jump label: update for .39
    On Thu, 2011-03-10 at 09:27 -0800, David Daney wrote:
    > On 03/10/2011 07:38 AM, Steven Rostedt wrote:

    > >> Can you explain what would prevent gcc from aligning these 3 pointers
    > >> (total of 24 bytes on 64-bit architectures) on 32-bytes ?
    > I can:
    > Section 3.1.2:
    > Aggregates and Unions

    Note, we are not dealing with C or arrays, but with inline assembly, and
    the linker.

    +static __always_inline bool arch_static_branch(struct jump_label_key *key)
    + asm goto("1:\tnop\n\t"
    + "nop\n\t"
    + ".pushsection __jump_table, \"aw\"\n\t"
    + WORD_INSN " 1b, %l[l_yes], %0\n\t"
    + ".popsection\n\t"
    + : : "i" (key) : : l_yes);
    + return false;
    + return true;

    That push/pop section part creates the structure we are talking about.
    It's made up of three pointers. The address of the nop, the address of
    the label l_yes and the address of the key.

    Now its up to the linker to decide where to place that element. Can we
    guarantee that it will always be on an 8 byte boundery?
    Hmm, I wonder if we could add a .ALIGN sizeof(long) before that?

    Now if we have two object files where there's a list of these jump
    labels, and then when the linker concatenates them we have something

    .long f1-a, .long f1-b, .long f1-c
    [ the above is 24 bytes ] so
    .long [ pad 8 bytes]
    .long f2-a, .long f2-b, .long f2-c ...

    Where f1 is object file 1 and f2 is object file 2. File 1 has a jump
    label table that holds a total of 24 bytes, and when the linker added
    the next jump label it padded it with 8 bytes into that section. The
    question remains, is that OK for the linker to do that, even though we
    specified in vmlinux.ld:

    /* implement dynamic printk debug */ \
    + . = ALIGN(8); \
    + VMLINUX_SYMBOL(__start___jump_table) = .; \
    + *(__jump_table) \
    + VMLINUX_SYMBOL(__stop___jump_table) = .; \

    But then again, maybe it will break on 32 bit, where the above file 1
    would have a total of 12 bytes, it may pad it with 4 bytes to keep that
    8 byte alignment.

    > Structures and unions assume the alignment of their most strictly
    > aligned component. Each member is assigned to the lowest
    > available offset with the appropriate alignment. The size of any
    > object is always a multiple of the object‘s alignment.
    > An array uses the same alignment as its elements, except that a
    > local or global array variable of length at least 16 bytes or a C99
    > variable-length array variable always has alignment of at least 16
    > bytes.
    > Structure and union objects can require padding to meet size and
    > alignment constraints. The contents of any padding is undefined.
    > I don't think it is explicitly stated, but it is also true that the size
    > is the smallest value that meets the above constraints.

    Could be true, but gcc has no idea that this data is an array. It's
    really up to the linker.

    > >> Also, could
    > >> you point out what would refrain the linker from aligning the start of
    > >> object sections on the next 32-bytes (thus power of two) address
    > >> multiple ?
    > >
    > The rules of the ABI are quite specific. It would be a toolchain bug if
    > this were messed up.
    > > Maybe it would be just easier to add another long ;)
    > Maybe we should audit all the data structures in the entire kernel and
    > add manual padding to power of 2 boundaries.

    We are not worried about normal C data structures, we are worried about
    data structures that are created by inline assembly and the linker. As
    we did have a bug with the trace_events code. But that dealt with a
    structure that was not strictly naturally word aligned. It had "int" as
    well as pointers.

    > >
    > > Seriously, it would. Then it would be 32 bytes on 64bit and 16 bytes on
    > > 32bit. Then I guess we can have our guarantee without doing a large
    > > change to have this indirect pointer and still waste sizeof(long) bytes
    > > in having it.
    > >
    > > Just insert a long "Reserved" word.
    > >
    > I disagree. Wasting memory to work around non-existent hypothetical
    > bugs seems wrong to me.

    The linker may never cause the issue. I haven't seen any problems with
    things that were naturally word aligned. But then, all the places that
    we do this has been naturally word aligned as well as a power of 2
    (extables for example).

    Thus, if we do "waste" space, I rather just add the 'Reserved' word and
    which makes it a power of 2 and be done with it.

    -- Steve

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