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

    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'

    > 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.


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


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