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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20130316183022.GB7560@redhat.com>
Date:	Sat, 16 Mar 2013 19:30:22 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	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 03/15, Paul E. McKenney wrote:
>
> On Fri, Mar 15, 2013 at 07:34:32PM +0100, Frederic Weisbecker wrote:
> > 2013/3/15 Oleg Nesterov <oleg@...hat.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.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/