linux-kernel - Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

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Date:	Fri, 7 Dec 2012 22:01:26 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tj@...nel.org, 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, sbw@....edu, amit.kucheria@...aro.org,
	rostedt@...dmis.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 v2 01/10] CPU hotplug: Provide APIs for "light"
	atomic readers to prevent CPU offline

On 12/07, Srivatsa S. Bhat wrote:
>
> CPU 0                         CPU 1
> ------                        ------
>
> 1. Acquire lock A             Increment CPU1's
>                               atomic counter
>
>
>
> 2. Increment CPU0's            Try to acquire lock A
>    atomic counter
>
>
> Now consider what happens if a hotplug writer (cpu_down) begins,

Exactly. So the fake lockdep_map should be per-cpu as well.

lglock doesn't need this because lg_local_lock() is not recursive
and lockdep can catch the bug like this. So it can look as single
lock for lockdep.

IOW. If you use the global lockdep_map and want lockdep to notice
this deadlock you need rwlock_acquire_read(map, 0, 0, IP). But then
lockdep will complain if the task does "read lock" under "read lock".

Oleg.

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