linux-kernel - Re: [PATCH] driver core / ACPI: Avoid device removal locking problems

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Date:	Wed, 28 Aug 2013 10:12:16 +0800
From:	Gu Zheng <guz.fnst@...fujitsu.com>
To:	Toshi Kani <toshi.kani@...com>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] driver core / ACPI: Avoid device removal locking problems

Hi Toshi,

On 08/28/2013 05:38 AM, Toshi Kani wrote:

> On Tue, 2013-08-27 at 17:21 +0800, Gu Zheng wrote:
>> Hi Rafael,
>>
>> On 08/26/2013 11:02 PM, Rafael J. Wysocki wrote:
>>
>>> On Monday, August 26, 2013 04:43:26 PM Rafael J. Wysocki wrote:
>>>> On Monday, August 26, 2013 02:42:09 PM Rafael J. Wysocki wrote:
>>>>> On Monday, August 26, 2013 11:13:13 AM Gu Zheng wrote:
>>>>>> Hi Rafael,
>>>
>>> [...]
>>>
>>>>
>>>> OK, so the patch below is quick and dirty and overkill, but it should make the
>>>> splat go away at least.
>>>
>>> And if this patch does make the splat go away for you, please also test the
>>> appended one (Tejun, thanks for the hint!).
>>>
>>> I'll address the ACPI part differently later.
>>
>> What about changing device_hotplug_lock and acpi_scan_lock to rwsem? like the
>> attached one(With a preliminary test, it also can make the splat go away).:)
> 
> I am curious how msleep(10) & restart_syscall() work in the change
> below.  Doesn't the msleep() make s_active held longer time, which can
> lead the thread holding device_hotplug_lock to wait it for deletion?

Yes, but it can avoid busy waiting. 

> Also, does restart_syscall() release s_active and reopen this file
> again?

Sure, it just set a TIF_SIGPENDING flag and return an -ERESTARTNOINTR error, s_active/file
will be released/closed in the failed path. And when do_signal() catches the -ERESTARTNOINTR,
it will change the regs to restart the syscall.

Thanks,
Gu

> 
> @@ -408,9 +408,13 @@ static ssize_t show_online(struct device *dev,
> struct device_attribute *attr,
>  {
>         bool val;
> 
> -       lock_device_hotplug();
> +       if (!read_lock_device_hotplug()) {
> +               msleep(10);
> +               return restart_syscall();
> +       }
> +
> 
> Thanks,
> -Toshi
> 
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