linux-kernel - Re: [PATCH 0/6] sched/cpusets fixes, more changes are needed

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: <20100324180912.GA21774@redhat.com>
Date:	Wed, 24 Mar 2010 19:09:12 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Ben Blum <bblum@...gle.com>,
	Jiri Slaby <jirislaby@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Miao Xie <miaox@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] sched/cpusets fixes, more changes are needed

On 03/24, Peter Zijlstra wrote:
>
> On Mon, 2010-03-15 at 10:09 +0100, Oleg Nesterov wrote:
> >
> > 	- do_fork() clears PF_STARTING and then calls wake_up_new_task()
> > 	  which finally does s/WAKING/RUNNING.
> >
> > 	  But. Nobody can take rq->lock in between. This means a signal
> > 	  from irq (quite possible with CLONE_THREAD) or another rt
> > 	  thread which preempts us can lockup.
>
> Hmm, the signal case might indeed be a problem, however I cannot see how
> the RT thread can be a problem because until we do wake_up_new_task()
> the child will not be runnable and can thus not be preempted.

Indeed, but I meant the _parent_ can be preempted ;)

In short. TASK_WAKING acts as a spinlock in fact. And since ttwu() can
be called from any context, it should be irq-safe: any owner must disable
inerrupts and preemption.

> The reason we have that TASK_WAKING stuff for fork is because
> wake_up_new_task() needs p->cpus_allowed to be stable

Sure! But it is very easy to change wake_up_new_task() to set TASK_WAKING
like ttwu() does. Of course, this needs raw_spin_lock_irq(rq->lock) for
a moment, but afaics that is all?

> So the below patch makes select_task_rq_fair unlock the rq when needed,
> and then puts all ->select_task_rq() calls under rq->lock. This should
> allow us to remove the TASK_WAKING thing from fork which in turn allows
> us to remove the PF_STARTING check in task_is_waking.
>
> How does that look?

I'll try to read this patch tomorrow. But could you please consider
the suggestion above?

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/