linux-kernel - Re: A possible sys

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Message-ID: <20100701140836.GA8379@redhat.com>
Date:	Thu, 1 Jul 2010 16:08:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Salman Qazi <sqazi@...gle.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Roland McGrath <roland@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: A possible sys_wait* bug

On 07/01, KOSAKI Motohiro wrote:
>
> > Basically, it is possibly for readers to continuously hold
> > tasklist_lock

Yes, this is the known problem.

Perhaps do_wait() is not the worst example. sys_kill(-1),
sys_ioprio_set() scan the global list.

> > I think the most direct approach to the problem is to have the
> > readers-writer locks be writer biased (i.e. as soon as a writer
> > contends, we do not permit any new readers).

I thought about this too, but this is deadlockable. At least,
read_lock(tasklist) should nest, and it should work in irq context.

We need the more fine-grained locking, but it is not clear to me what
should be done in the long term. Afaics, this is very nontrivial.

Oleg.

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