Re: +atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive.patch added to -mm tree

From: Oleg Nesterov
Date: Thu Mar 21 2013 - 14:32:36 EST

On 03/21, Eric Dumazet wrote:
> On Thu, 2013-03-21 at 18:08 +0100, Oleg Nesterov wrote:
> > OK... since nobody volunteered to make a patch, what do you think about
> > the change below?
> >
> > It should "fix" atomic_add_unless() (only on x86) and optimize
> > atomic_inc/dec_unless.
> >
> > With this change atomic_*_unless() can do the unnecessary mb() after
> > cmpxchg() fails, but I think this case is very unlikely.
> >
> > And, in the likely case atomic_inc/dec_unless avoids the 1st cmpxchg()
> > which in most cases just reads the memory for the next cmpxchg().
> >
> > Oleg.
> Hmm, cmpxchg() has different effect on MESI transaction, than a plain
> read.

But this doesn't matter?

We will do cmpxchg() anyway. Unless we can see that it will fail.

Or could you explain what I missed?

> maybe the 'hint' idea used in atomic_inc_not_zero_hint() could be used.

To me, it would be better to kill atomic_inc_not_zero_hint() or unify
unify it with atomic_inc_not_zero(). But this is another story.


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