linux-kernel - Re: + atomic-improve-atomic_inc_unless_negative-atomic_dec_unless

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Date:	Thu, 21 Mar 2013 19:30:21 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ming Lei <tom.leiming@...il.com>, Shaohua Li <shli@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: +
	atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive
	.patch added to -mm tree

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.

Oleg.

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