Re: Towards eliminating the freezer

Rafael J. Wysocki
2007-07-24T09:26:01Z

On Monday, 23 July 2007 22:05, Alan Stern wrote:
> [Note changed $SUBJECT]
[--snip--]
> =============================
>
>
> 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
cycle.

Greetings,
Rafael

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