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    SubjectRe: [RFC][PATCH 0/5] arch: atomic rework
    Hi,

    On Mon, 24 Feb 2014, Linus Torvalds wrote:

    > > To me that reads like
    > >
    > > int i;
    > > int *q = &i;
    > > int **p = &q;
    > >
    > > atomic_XXX (p, CONSUME);
    > >
    > > orders against accesses '*p', '**p', '*q' and 'i'. Thus it seems they
    > > want to say that it orders against aliased storage - but then go further
    > > and include "indirectly through a chain of pointers"?! Thus an
    > > atomic read of a int * orders against any 'int' memory operation but
    > > not against 'float' memory operations?
    >
    > No, it's not about type at all, and the "chain of pointers" can be
    > much more complex than that, since the "int *" can point to within an
    > object that contains other things than just that "int" (the "int" can
    > be part of a structure that then has pointers to other structures
    > etc).

    So, let me try to poke holes into your definition or increase my
    understanding :) . You said "chain of pointers"(dereferences I assume),
    e.g. if p is result of consume load, then access to
    p->here->there->next->prev->stuff is supposed to be ordered with that load
    (or only when that last load/store itself is also an atomic load or
    store?).

    So, what happens if the pointer deref chain is partly hidden in some
    functions:

    A * adjustptr (B *ptr) { return &ptr->here->there->next; }
    B * p = atomic_XXX (&somewhere, consume);
    adjustptr(p)->prev->stuff = bla;

    As far as I understood you, this whole ptrderef chain business would be
    only an optimization opportunity, right? So if the compiler can't be sure
    how p is actually used (as in my function-using case, assume adjustptr is
    defined in another unit), then the consume load would simply be
    transformed into an acquire (or whatever, with some barrier I mean)? Only
    _if_ the compiler sees all obvious uses of p (indirectly through pointer
    derefs) can it, yeah, do what with the consume load?


    Ciao,
    Michael.


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