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    SubjectRe: [PATCH] kernel: fix data race in put_pid
    On 09/18, Dmitry Vyukov wrote:
    >
    > On Fri, Sep 18, 2015 at 3:44 PM, Oleg Nesterov <oleg@redhat.com> wrote:
    > > So I assume that if we have
    > >
    > > int X = 0;
    > > atomic_t Y = ATOMIC_INIT(0);
    > >
    > > void w(void)
    > > {
    > > X = 1;
    > > atomic_inc_return(&Y);
    > > }
    > >
    > > then
    > >
    > > void r(void)
    > > {
    > > if (atomic_read_ctrl(&Y))
    > > BUG_ON(X == 0);
    > > }
    > >
    > > should be correct? Why?
    > >
    > > If not then I am even more confused.
    >
    > This not correct,

    Good. because I wasn't able to understand why this could work.

    > // thread 1
    > X = 1;
    > atomic_inc_return(&Y);
    >
    > // thread 2
    > if (atomic_read_ctrl(&Y)) {
    > X = 2;
    > BUG_ON(X == 2);
    > }

    Thanks. This makes perfect sense to me.

    And then I agree, atomic_read_ctrl() in put_pid() should fix the
    theoretical problem.

    Perhaps we can add this example to memory-barriers.txt... Although
    perhaps it already explains/documents this case. I am afraid to open
    it, it is huge and changes too often so every time it looks like a
    new document to me ;)

    Oleg.



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