linux-kernel - Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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Message-ID: <20121209205733.GA7038@redhat.com>
Date:	Sun, 9 Dec 2012 21:57:33 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	rostedt@...dmis.org
Cc:	tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
	amit.kucheria@...aro.org, rjw@...k.pl, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU
	offline from atomic context

On 12/07, Srivatsa S. Bhat wrote:
>
> 4. No deadlock possibilities
>
>    Per-cpu locking is not the way to go if we want to have relaxed rules
>    for lock-ordering. Because, we can end up in circular-locking dependencies
>    as explained in https://lkml.org/lkml/2012/12/6/290

OK, but this assumes that, contrary to what Steven said, read-write-read
deadlock is not possible when it comes to rwlock_t. So far I think this
is true and we can't deadlock. Steven?

However. If this is true, then compared to preempt_disable/stop_machine
livelock is possible. Probably this is fine, we have the same problem with
get_online_cpus(). But if we can accept this fact I feel we can simmplify
this somehow... Can't prove, only feel ;)

Oleg.

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