linux-kernel - Re: [RFC PATCH v5 1/2] Use lock_device_hotplug() in cpu_probe_store() and cpu_release

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Date:	Tue, 22 Apr 2014 10:29:37 +0800
From:	Li Zhong <zhong@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, gregkh@...uxfoundation.org,
	rafael.j.wysocki@...el.com, toshi.kani@...com
Subject: Re: [RFC PATCH v5 1/2] Use lock_device_hotplug() in
 cpu_probe_store() and cpu_release_store()

On Mon, 2014-04-21 at 18:38 -0400, Tejun Heo wrote:
> Hello,
> 
> On Mon, Apr 21, 2014 at 05:20:59PM +0800, Li Zhong wrote:
> > While auditing the usage of lock_device_hotplug_sysfs() for implementing
> > it in another way in following patch, it seems to me that the code here
> > is to add/remove device, and the files probe/release for cpu bus
> > themselves won't be removed. 
> > 
> > So it seems to me there is no s_active related deadlock here, and we
> > could just use lock_device_hotplug().
> 
> It may still cause issue if offlining ends up removing sysfs files or
> gets involved with the same lock used during cpu hot[un]plug
> operations (e.g. offlining racing against cpu hotunplug) and offlining
> a cpu does remove files.  Has this change been tested?

The probe/release files are attribute files for cpu subsys, looks like
following in sysfs dirs

$ cd /sys/devices/system/cpu/
$ ls -l
total 0
drwxr-xr-x. 7 root root     0 Apr 17 19:00 cpu0
drwxr-xr-x. 4 root root     0 Apr 17 19:00 cpu1
drwxr-xr-x. 4 root root     0 Apr 17 19:00 cpu10
......
drwxr-xr-x. 3 root root     0 Apr 20 08:00 cpufreq
drwxr-xr-x. 2 root root     0 Apr 20 08:00 cpuidle
-rw-------. 1 root root 65536 Apr 21 00:28 dscr_default
-r--r--r--. 1 root root 65536 Apr 21 00:28 kernel_max
-r--r--r--. 1 root root 65536 Apr 21 00:28 offline
-r--r--r--. 1 root root 65536 Sep  4  2014 online
-r--r--r--. 1 root root 65536 Apr 21 00:28 possible
drwxr-xr-x. 2 root root     0 Apr 20 08:00 power
-r--r--r--. 1 root root 65536 Apr 17 20:46 present
--w-------. 1 root root 65536 Apr 21 00:28 probe        <-----	
--w-------. 1 root root 65536 Apr 21 00:28 release      <-----
-rw-------. 1 root root 65536 Apr 21 00:28 subcores_per_core
-rw-r--r--. 1 root root 65536 Apr 21 00:28 uevent

>From the code, it seems cpu subsys won't be unregistered, and it doesn't
make sense to remove all the cpus in the system. 

Thanks, Zhong

> 
> Thanks.
> 


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