linux-kernel - Re: [BUGFIX v2 2/4] ACPI, DOCK: resolve possible deadlock scenarios

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Message-ID: <2972274.Y9Nv7DUfsQ@vostro.rjw.lan>
Date:	Mon, 17 Jun 2013 13:40:32 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jiang Liu <liuj97@...il.com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Yinghai Lu <yinghai@...nel.org>,
	"Alexander E . Patrakov" <patrakov@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Yijing Wang <wangyijing@...wei.com>,
	linux-acpi@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
	stable@...r.kernel.org, Jiang Liu <jiang.liu@...wei.com>
Subject: Re: [BUGFIX v2 2/4] ACPI, DOCK: resolve possible deadlock scenarios

On Monday, June 17, 2013 01:12:00 AM Jiang Liu wrote:
> On 06/16/2013 06:54 AM, Rafael J. Wysocki wrote:
> > On Saturday, June 15, 2013 11:20:40 PM Rafael J. Wysocki wrote:
> >> On Saturday, June 15, 2013 10:17:42 PM Rafael J. Wysocki wrote:
> > 
> > [...]
> > 
> >>
> >> Which sysfs interfaces do you mean, by the way?
> >>
> >> If you mean "eject", then it takes acpi_scan_lock and hotplug_dock_devices()
> >> should always be run under acpi_scan_lock too.  It isn't at the moment,
> >> because write_undock() doesn't take acpi_scan_lock(), but this is an obvious
> >> bug (so I'm going to send a patch to fix it in a while).
> >>
> >> With that bug fixed, the possible race between acpi_eject_store() and
> >> hotplug_dock_devices() should be prevented from happening, so perhaps we're
> >> worrying about something that cannot happen?
> > 
> > So here's a question: What particular races are possible if we remove
> > ds->hp_lock entirely without doing anything else just yet?  I mean, how to
> > *trigger* them from the start to the end and not how they can possibly happen
> > but never do, because there's no way they can be actually triggered?
> Hi Rafael,
>     I have no really platform which triggers this bug, but I may imagine
> a possible platform if it's valid for explanation.
>     Let's think about a laptop dock station with a thunderbolt
> controller built-in. The dock station is managed by dock driver and
> acpiphp driver. And the PCIe hierarchy managed by the thunderbolt
> controller may be managed by dock driver and ACPIPHP driver too.
> So it may trigger the issue by pressing the dock button and unplugging
> thunderbolt cable concurrently.
>     But after all, this is all by imagination:). We may need to find a
> simple and quick solution for 3.10 and the stable trees and enhance the
> solution later to avoid introducing new bugs while fixing a bug.

Well, if that particular bug is not fixed in 3,10, it won't be a tragedy,
and I *really* don't want that code to get substantially more complex than
it is now.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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