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    SubjectRe: + kthread-add-a-missing-memory-barrier-to-kthread_stop.patch added to -mm tree
    On 23/02/2008, Linus Torvalds <torvalds@linux-foundation.org> wrote:
    >
    >
    > On Sat, 23 Feb 2008, Oleg Nesterov wrote:
    > >
    >
    > > Yes, but still I suspect wmb() is not enough. Note that try_to_wake_up()
    > > first checks (reads) the task->state,
    > >
    > > if (!(old_state & state))
    > > goto out;
    > >
    > > without the full mb() it is (in theory) possible that try_to_wake_up()
    > > first reads TASK_RUNNING and only then sets CONDITION. IOW, STORE and
    > > LOAD could be re-ordered.
    >
    >
    > No. The spinlock can have preceding stores (and loads, for that matter)
    > percolate *into* the locked region, but a spinlock can *not* have loads
    > (and stores) escape *out* of the region withou being totally broken.

    it's not a LOAD that escapes *out* of the region. It's a MODIFY that gets *in*:

    (1)

    MODIFY(a);

    LOCK

    LOAD(b);

    UNLOCK


    can become:

    (2)

    LOCK

    MOFIDY(a)
    LOAD(b);

    UNLOCK

    and (reordered)

    (3)

    LOCK

    LOAD(a)
    MODIFY(b)

    UNLOCK

    and this last one is a problem. No?


    >
    > Linus
    >

    --
    Best regards,
    Dmitry Adamushko


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