netdev - Re: [PATCH v6 08/46] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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]

Date:	Tue, 19 Feb 2013 17:40:50 +0800
From:	Michel Lespinasse <walken@...gle.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	rostedt@...dmis.org, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, rjw@...k.pl, sbw@....edu,
	fweisbec@...il.com, linux@....linux.org.uk,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	vincent.guittot@...aro.org
Subject: Re: [PATCH v6 08/46] CPU hotplug: Provide APIs to prevent CPU offline
 from atomic context

On Tue, Feb 19, 2013 at 2:50 AM, Srivatsa S. Bhat
<srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> But, the whole intention behind removing the parts depending on the
> recursive property of rwlocks would be to make it easier to make rwlocks
> fair (going forward) right? Then, that won't work for CPU hotplug, because,
> just like we have a legitimate reason to have recursive
> get_online_cpus_atomic(), we also have a legitimate reason to have
> unfairness in locking (i.e., for deadlock-safety). So we simply can't
> afford to make the locking fair - we'll end up in too many deadlock
> possibilities, as hinted in the changelog of patch 1.

Grumpf - I hadn't realized that making the underlying rwlock fair
would break your hotplug use case. But you are right, it would. Oh
well :/

> So the only long-term solution I can think of is to decouple
> percpu-rwlocks and rwlock_t (like what Tejun suggested) by implementing
> our own unfair locking scheme inside. What do you think?

I have no idea how hard would it be to change get_online_cpus_atomic()
call sites so that the hotplug rwlock read side has a defined order vs
other locks (thus making sure the situation you describe in patch 1
doesn't happen). I agree we shouldn't base our short term plans around
that, but maybe that's doable in the long term ???

Otherwise, I think we should add some big-fat-warning that percpu
rwlocks don't have reader/writer fairness, that the hotplug use case
actually depends on the unfairness / would break if the rwlock was
made fair, and that any new uses of percpu rwlocks should be very
carefully considered because of the reader/writer fairness issues.
Maybe even give percpu rwlocks a less generic sounding name, given how
constrained they are by the hotplug use case.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html