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    SubjectRe: [linux-pm] Fundamental flaw in system suspend, exposed by freezer removal
    On Monday, 25 of February 2008, Alan Stern wrote:
    > On Mon, 25 Feb 2008, Alan Stern wrote:
    > > The only possible solution is to have the drivers themselves be
    > > responsible for preventing calls to device_add() or device_register()
    > > during a system sleep. (It's also necessary to prevent driver binding,
    > > but this isn't a major issue.) The most straightforward approach is to
    > > add a new pair of driver methods: one to disable adding children and
    > > one to re-enable it. Of course this would represent a significant
    > > addition to the Power Management driver interface.
    > >
    > > (Note that the existing suspend and resume methods cannot be used for
    > > this purpose. Drivers assume that when the suspend method is called,
    > > it has already been called for all the child devices. This wouldn't be
    > > true if one of the purposes of the method was to prevent addition of
    > > new children.)
    > On further thought maybe the existing methods can be used, with care.
    > Drivers would have to assume the responsibility of synchronizing with
    > their helper threads and stopping addition of new children (something
    > they should already be doing), and they would also have to check that
    > all the existing children are already suspended. They should not make
    > the assumption that the PM core has already suspended all the children.

    IMO the device driver should assure that no new children will be registered
    concurrently with the ->suspend() method (IOW, ->suspend() should wait for
    all such registrations to complete and should prevent any new ones from
    being started) and it should make it impossible to register any new children
    after ->suspend() has run. It's the driver's problem how to achieve that.

    > The PM core could help detect errors here. If it tries to suspend a
    > device and sees that the device's parent is already suspended, then the
    > parent's driver has a bug.

    Yes, I think we ought to fail the suspend in such cases. Still, that's not
    sufficient to prevent a child from being registered after we've run
    dpm_suspend(). For this reason, we could also leave dpm_suspend() with
    dpm_list_mtx held and not release it until the next dpm_resume() is run.

    That will potentially cause some trouble to CPU hotplug cotifiers, but we can
    handle that, for example, by using the in_suspend_context() test.


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