2009/8/8 Alan Stern [off-list ref]:

On Fri, 7 Aug 2009, Peter Zijlstra wrote:

quoted

It used to be that _all_ dev->sem instances were taken on suspend or
something like that, I think that got fixed a long while back.

I'd have to look at what the current locking requirements for dev->sem
are.

It is supposed to be locked whenever the driver core invokes a probe,
remove, or PM-related callback.  Under some circumstances, the parent's
semaphore is supposed to be locked as well.  Individual subsystems may
have their own requirements in addition to these.

The ordering requirement is: Don't try to acquire a device's lock if
you already hold the lock for a non-ancestor device.  More generally
(if more obscurely): If you already hold device A's lock, then don't
try to acquire the lock for device B unless you already hold the lock
for A & B's most recent common ancestor.

It seems that the following case is very common, and A and B have no
common ancestor, but we can hold device A and B's lock at the same
time, can't we?

Thanks.

device A  comes in one bus:
	device_add()
         ->bus_attach_device()
            ->device_attach():drivers/base/dd.c /*holding device A's lock*/
               ->...drv->probe()		/*sleep here some time*/

then device B comes in another bus:
	device_add()
         ->bus_attach_device()
            ->device_attach():drivers/base/dd.c /*holding device B's lock*/
               ->...drv->probe()		/*sleep here some time*/

-- 
Lei Ming