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    SubjectRe: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)
    Date
    On Tuesday 08 December 2009, Alan Stern wrote:
    > On Tue, 8 Dec 2009, Linus Torvalds wrote:
    >
    > > 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.
    >
    > Well, one difficulty. It arises only because we are contemplating
    > having the PM core fire up the async tasks, rather than having the
    > drivers' suspend routines launch them (the way your original proposal
    > did -- the difficulty does not arise there).
    >
    > Suppose A and B are unrelated devices and we need to impose the
    > off-tree constraint that A suspends after B. With children taking
    > their parent's lock, the way to prevent A from suspending too soon is
    > by having B's suspend routine acquire A's lock.
    >
    > But B's suspend routine runs entirely in an async task, because that
    > task is started by the PM core and it does the method call. Hence by
    > the time B's suspend routine is called, A may already have begun
    > suspending -- it's too late to take A's lock. To make the locking
    > work, B would have to acquire A's lock _before_ B's async task starts.
    > Since the PM core is unaware of the off-tree dependency, there's no
    > simple way to make it work.

    Do not set async_suspend for B and instead start your own async thread
    from its suspend callback. The parent-children synchronization is done by the
    core anyway (at least I'd do it that way), so the only thing you need to worry
    about is the extra dependency.

    > > That just complicates things. Compare to my simple locking scheme I've
    > > quoted several times.
    >
    > It is a little more complicated in that it involves explicitly
    > iterating over children. But it is simpler in that it can use
    > completions instead of rwsems and it avoids the off-tree dependency
    > problem described above.

    I would be slightly more comfortable using completions, but the rwsem-based
    approach is fine with me as well.

    Rafael


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