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

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Date:	Wed, 28 Aug 2013 10:55:41 -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 Wed, 2013-08-28 at 10:12 +0800, Gu Zheng wrote:
> On 08/28/2013 05:38 AM, Toshi Kani wrote:
 :
> >>
> >> 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. 

I know, but it's kinda unfortunate to sleep with s_active held in this
situation.  But I am fine with the 5ms Rafael used in this latest
patchset since this is a rare case anyway.

> > 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.

I see.  This is a clever functionality. 

Thanks,
-Toshi


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