Messages in this thread | | | Date | Sat, 16 Mar 2013 19:30:22 +0100 | From | Oleg Nesterov <> | Subject | Re: + atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive .patch added to -mm tree |
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On 03/15, Paul E. McKenney wrote: > > On Fri, Mar 15, 2013 at 07:34:32PM +0100, Frederic Weisbecker wrote: > > 2013/3/15 Oleg Nesterov <oleg@redhat.com>: > > > > > > 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. > > Completely agreed. It is not like memory ordering is simple, so we should > keep the rules simple.
It is hardly possible to argue with this ;)
> The rule is that if an atomic primitive returns non-void, then there is > a full memory barrier before and after.
This case is documented...
> This applies to primitives > returning boolean as well, with atomic_dec_and_test() setting this > precedent from what I can see.
I don't think this is the "fair" comparison. Unlike atomic_add_unless(), atomic_dec_and_test() always changes the memory even if it "fails".
If atomic_add_unless() returns 0, nothing was changed and if we add the barrier it is not clear what it should be paired with.
But OK. I have to agree that "keep the rules simple" makes sense, so we should change atomic_add_unless() as well.
Oleg.
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