linux-kernel - Re: [RFC PATCH v3] Use kernfs_break_active_protection() for device online store callbacks

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Date:	Tue, 15 Apr 2014 10:50:17 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, gregkh@...uxfoundation.org,
	rafael.j.wysocki@...el.com, toshi.kani@...com
Subject: Re: [RFC PATCH v3] Use kernfs_break_active_protection() for device
 online store callbacks

Hello,

On Tue, Apr 15, 2014 at 10:44:37AM +0800, Li Zhong wrote:
> / *
>   * This process might deadlock with another process trying to 
>   * remove this device:
>   * This process holding the s_active of "online" attribute, and tries 
>   * to online/offline the device with some locks protecting hotplug.
>   * Device removing process holding some locks protecting hotplug, and 
>   * tries to remove the "online" attribute, waiting for the s_active to
>   * be released. 
>   *
>   * The deadlock described above should be solved with
>   * lock_device_hotplug_sysfs(). We temporarily drop the active 
>   * protection here to avoid some lockdep warnings. 
>   *
>   * If device_hotplug_lock is forgotten to be used when removing
>   * device(possibly some very simple device even don't need this lock?),
>   * @dev could go away any time after dropping the active protection. 
>   * So increase its ref count before dropping active protection. 
>   * Though invoking device_{on|off}line() on a removed device seems
>   * unreasonable, it should be less disastrous than playing with freed
>   * @dev. Also, we might be able to have some mechanism abort 
>   * device_{on|off}line() if @dev already removed.
>   */

Hmmm... I'm not sure I fully understand the problem.  Does the code
ever try to remove "online" while holding cpu_add_remove_lock and,
when written 0, online knob grabs cpu_add_remove_lock?  If so, that is
an actually possible deadlock, no?

Thanks.

-- 
tejun
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