On 03/15, Frederic Weisbecker wrote: > > 2013/3/15 Oleg Nesterov <o...@redhat.com>: > > > > do_something() looks fine, if atomic_add_unless_negative() succeeds > > we do have a barrier? > > Ok, I guess the guarantee of a barrier in case of failure is probably > not needed. But since the only way to safely read the atomic value is > a cmpxchg like operation, I guess a barrier must be involved in any > case. > > Using atomic_read() may return some stale value.
Oh, if the lack of the barrier is fine, then "stale" should be fine too, I think. I bet you can't describe accurately what "stale" can actually mean in this case ;) If, say, atomic_inc_unless_negative(p) sees the stale value < 0, it was actually negative somewhere in the past. If it was changed later, we can pretend that atomic_inc_unless_negative() was called before the change which makes it positive. > > Anyway, I understand that it is possible to write the code which > > won't work without the uncoditional mb(). > > Yeah that's my fear. I see... well personally I can't imagine the "natural" (non-artificial) code example which needs mb() in case of failure. However, I have to agree with Paul's "It is not like memory ordering is simple", so I won't argue. > > My point was: should we fix atomic_add_unless() then? If not, why > > should atomic_add_unless_negative() differ? > > They shouldn't differ I guess. Agreed, they shouldn't. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/