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    SubjectRe: + atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive .patch added to -mm tree
    On 03/15, Ming Lei wrote:
    >
    > On Fri, Mar 15, 2013 at 9:46 PM, Oleg Nesterov <oleg@redhat.com> wrote:
    > > On 03/15, Ming Lei wrote:
    > >>
    > >> On Fri, Mar 15, 2013 at 12:24 AM, Oleg Nesterov <oleg@redhat.com> wrote:
    > >> > static inline int atomic_inc_unless_negative(atomic_t *p)
    > >> > {
    > >> > int v, v1;
    > >> > - for (v = 0; v >= 0; v = v1) {
    > >> > + for (v = atomic_read(p); v >= 0; v = v1) {
    > >> > v1 = atomic_cmpxchg(p, v, v + 1);
    > >>
    > >> Unfortunately, the above will exchange the current value even though
    > >> it is negative, so it isn't correct.
    > >
    > > Hmm, why? We always check "v >= 0" before we try to do
    > > atomic_cmpxchg(old => v) ?
    >
    > Sorry, yes, you are right. But then your patch is basically same with the
    > previous one, isn't it?

    Sure, the logic is the same, just the patch (and the code) looks simpler
    and more understandable.

    > And has same problem, see below discussion:
    >
    > http://marc.info/?t=136284366900001&r=1&w=2

    The lack of the barrier?

    I thought about this, this should be fine? atomic_add_unless() has the same
    "problem", but this is documented in atomic_ops.txt:

    atomic_add_unless requires explicit memory barriers around the operation
    unless it fails (returns 0).

    I thought that atomic_add_unless_negative() should have the same
    guarantees?

    Paul? Frederic?

    Oleg.



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