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

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Message-ID: <20140417150640.GJ15326@htj.dyndns.org>
Date:	Thu, 17 Apr 2014 11:06:40 -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 Thu, Apr 17, 2014 at 11:05:53AM +0800, Li Zhong wrote:
> It seems to me cpu_add_remove_lock is always taken after
> device_hotplug_lock.
> 
> So if cpu_add_remove_lock has been acquired by device removing process,
> then it means the other online/offline process couldn't successfully try
> lock device_hotplug_lock, and will release s_active with a restart
> syscall error;
> 
> if cpu_add_remove_lock has been acquired by online/offline process, then
> it should already hold device_hotlug_lock, and keeps the device removing
> process waiting at device_hotplug_lock. So online/offline process could
> release the lock, and finally release s_active soon. 

I see.  That's kinda nasty tho and lockdep of course doesn't know
about it and generates spurious warnings.

> But after some further thinking, I seem to understand your point.
> s_active has lock order problem with the other series of hotplug related
> locks, so it's better to take s_active out of the dependency chain,
> rather than the first of the other series of locks? like you suggested
> below.

Yeah, I think that'd be the right thing to do and we can revert the
previous convolution.

Thanks.

-- 
tejun
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