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Message-ID: <Pine.LNX.4.44L0.0801071412530.6739-100000@iolanthe.rowland.org>
Date: Mon, 7 Jan 2008 14:29:57 -0500 (EST)
From: Alan Stern <stern@...land.harvard.edu>
To: "Rafael J. Wysocki" <rjw@...k.pl>
cc: Johannes Berg <johannes@...solutions.net>,
Greg KH <gregkh@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Len Brown <lenb@...nel.org>, Ingo Molnar <mingo@...e.hu>,
ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
pm list <linux-pm@...ts.linux-foundation.org>
Subject: Re: [PATCH] PM: Acquire device locks on suspend
On Mon, 7 Jan 2008, Rafael J. Wysocki wrote:
> On Monday, 7 of January 2008, Alan Stern wrote:
> > On Mon, 7 Jan 2008, Rafael J. Wysocki wrote:
> >
> > > Please see the patch at: http://lkml.org/lkml/2008/1/6/298 . It represents my
> > > current idea about how to do that.
> >
> > It has some problems.
> >
> > First, note that the list manipulations in dpm_suspend(),
> > device_power_down(), and so on aren't protected by dpm_list_mtx. So
> > your patch could corrupt the list pointers.
>
> Yes, they need the locking. I have overlooked that, mostly because the locking
> was removed by gregkh-driver-pm-acquire-device-locks-prior-to-suspending.patch
> too (because you assumed there woundn't be any need to remove a device during
> a suspend, right?).
Right.
> > Are you assuming that no other threads can be running at this time?
>
> No, I'm not.
>
> > Note also that device_pm_destroy_suspended() does up(&dev->sem), but it
> > doesn't know whether or not dev->sem was locked to begin with.
>
> Do you mean it might have been released already by another thread
> calling device_pm_destroy_suspended() on the same device?
I was thinking that it might be called before lock_all_devices().
However let's ignore that possibility and simplify the discussion by
assuming that destroy_suspended_device() is never called except by a
suspend or resume method for that device or one of its ancestors.
(This still leaves the possibility that it might get called by mistake
during a runtime suspend or resume...)
> > Do you want to rule out the possibility of a driver's suspend or remove
> > methods calling destroy_suspended_device() on its own device? With
> > your synchronous approach, this would mean that the suspend/resume
> > method would indirectly end up calling the remove method. This is
> > dangerous at best; with USB it would be a lockdep violation. With an
> > asynchronous approach, on the other hand, this wouldn't be a problem.
>
> Well, the asynchronous apprach has the problem that the device may end up
> on a wrong list when removed by one of the .suspend() callbacks (and I don't
> see how to avoid that without extra complexity). Perhaps that's something we
> can live with, though.
The same problem affects the synchronous approach. We can fix it by
having dpm_suspend() do the list_move() before calling
suspend_device(). Then if the suspend fails move the device back.
> One more question: is there any particular reason not to call
> device_pm_remove() at the beginning of device_del()?
I think it's done this way to avoid having a window where the device
isn't on a PM list and is still owned by the bus and the driver. But
if a suspend occurs during that window, it shouldn't matter that the
device will be left unsuspended. After all, the same thing would have
happened if the suspend occurred after bus_remove_device().
So no, there shouldn't be a problem with moving the call.
Alan Stern
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