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    SubjectRe: [PATCH 0/2] jump label: update for .39
    * David Daney ( wrote:
    > On 03/10/2011 07:38 AM, Steven Rostedt wrote:
    >> On Thu, 2011-03-10 at 09:11 -0500, Mathieu Desnoyers wrote:
    >>> * Steven Rostedt ( wrote:
    >>>> On Wed, 2011-03-09 at 15:47 -0500, Jason Baron wrote:
    >>>>> Hi,
    >>>>> Re-fresh of updates against latest -tip tree.
    >>>> Thanks Jason,
    >>>> I started looking at them, I should have comments tomorrow (if I have
    >>>> any comments ;)
    >>>>> I've tried to split this update up somewhat, but I've only succeeded to split
    >>>>> out the dynamic debug bits. The interface changes and re-write are quite
    >>>>> intertwined.
    >>>>> I believe this update should address all the comments from the previous posting
    >>>>> except for Mathieu's request for a section of jump label pointers that point to
    >>>>> the jump label structures (since the compiler might leave gaps in the jump label
    >>>>> structures).
    >>>> The jump label structures is a list of 3 pointers, correct? I doubt that
    >>>> gcc would place any holes in it as they are all aligned by natural word
    >>>> size.
    >>> Hi Steven,
    >>> 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
    > 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.

    Your quote seems to apply in this case because:

    a) the "structure" is defined in assembly, so the compiler is not
    deciding the alignment of the structure.

    b) the C code that iterates on the section array in jump_label.c see an
    array of structures, which imply that gcc does not have no freedom in
    terms of up-aligning these structures.

    The problem we faced with trace event and tracepoints was caused by the
    fact that we did define static structures from within C code, and the
    compiler mistakenly believed that it was the only one holding references
    to these structures, so it could up-align them to larger multiples as it
    wants, because they were each independent objects.

    >>> 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.

    The trace event and tracepoint code are doing static structure
    declarations into specific sections for the linker to create array
    bounds with, and this last part sadly falls outside of the compiler
    knowledge, and therefore might be outside of the ABI scope because the
    compiler believes that these objects are local to the object and never
    ever exported. Note that up-aligning on larger multiples still meets the
    local alignment requirements -- it's just not valid to e.g. create an
    array that assumes that these statically defined structures are next to
    another without any extra padding.

    >> 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.

    Only required when we play games with sections outside of the compiler
    knowledge, which is really a handful of cases.

    >> 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.

    This object layout problem has been me with tracepoints in the past, and
    has bitter Steven with trace event recently. It is therefore very real.



    > David Daney

    Mathieu Desnoyers
    Operating System Efficiency R&D Consultant
    EfficiOS Inc.
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