Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 8 Dec 2009, Alan Stern wrote:
> 
> That's not the way it should be done.  Linus had children taking their
> parents' locks during suspend, which is simple but leads to
> difficulties.

No it doesn't. Name them.

> Instead, the PM core should do a down_write() on each device before
> starting the device's async suspend routine, and an up_write() when the
> routine finishes.

No you should NOT do that. If you do that, you serialize the suspend 
incorrectly and much too early. IOW, think a topology like this:

	a -> b -> c
	  \
	   > d -> e

where you'd want to suspend 'c' and 'e' asynchronously.  If we do a 
'down-write()' on b, then we'll delay until 'c' has suspended, an if we 
have ordered the nodes in the obvious depth-first order, we'll walk the PM 
device list in the order:

	c b e d a

and now we'll serialize on 'b', waiting for 'c' to suspend. Which we do 
_not_ want to do, because the whole point was to suspend 'c' and 'e' 
together.

> Parents should, at the start of their async routine,
> do down_read() on each of their children plus whatever other devices
> they need to wait for.  The core can do the waiting for children part 
> and the driver's suspend routine can handle any other waiting.

Why?

That just complicates things. Compare to my simple locking scheme I've 
quoted several times. 

		Linus