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    SubjectRe: Towards eliminating the freezer
    On Monday, 23 July 2007 22:05, Alan Stern wrote:
    > [Note changed $SUBJECT]
    > =============================
    > Now here's an idea which might work. Can we require every caller of
    > device_add() to hold some existing device's semaphore? Normally it
    > would be the semaphore of the new device's parent, but it could be a
    > higher ancestor. There even could be a single "root" semaphore for
    > drivers registering a top-level device with no parent.
    > (Some testing shows that during startup things like ACPI and IDE don't
    > fulfill this requirement, so maybe we should require it only after
    > userspace has begun running. After all, the system can't suspend
    > until then.)
    > It seems like a reasonable sort of thing to do. Hotplugged devices
    > tend to be registered as they are discovered by their parent's driver,
    > so it shouldn't be too much to ask that the parent's semaphore be held
    > when the new device is registered. Static devices generally aren't
    > quite so nice; the serial and floppy drivers in particular would need a
    > little work (and probably some other drivers too).
    > If we do this, then once the PM core has acquired the semaphore for
    > every device it will be guaranteed that no new devices can be added.
    > It would be a simple solution to a rather nasty problem.

    Hmm, in device_pm_add() and device_pm_remove() we acquire dpm_list_mtx which
    also is acquired by device_suspend() and device_resume(). Thus, every attempt
    to register a new device or unregister an existing one will be blocked while
    either device_suspend() or device_resume() is running.

    If we arrange things so that dpm_list_mtx is acquired, but not released, by
    device_suspend() and released, but not acquired, by device_resume(), then it
    won't be possible to register/unregister a device during a suspend-resume


    "Premature optimization is the root of all evil." - Donald Knuth
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