Re: [patch]a lockless method to alter pm count

Am Dienstag, 24. April 2007 21:47 schrieb Alan Stern:
> On Tue, 24 Apr 2007, Oliver Neukum wrote:
> 
> > Hi,
> > 
> > some drivers'll need a lockless method to alter the pm count in order
> > to take locks by themselves in correct order.
> 
> (Minor comment: Acquiring the lock yourself to alter the pm count isn't
> quite the same as doing it locklessly.)
> 
> Can you give an example where this would be needed?  I can't think of one 
> offhand.

Certainly. I need this to do autosuspend for the storage driver.

usb_suspend()
usb_pm_lock(udev)
usb_suspend_both(udev)
storage:
storage_suspend()
mutex_lock(&us->dev_mutex);

The storage thread:
usb_stor_control_thread()
mutex_lock(&(us->dev_mutex));
usb_autopm_get_interface()

AB <-> BA deadlock

	Regards
		Oliver

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