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

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Date:	Tue, 27 Aug 2013 15:38:49 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Gu Zheng <guz.fnst@...fujitsu.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

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

@@ -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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