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    SubjectRe: [PATCH] PM: Prevent waiting forever on asynchronous resume after abort
    On Friday, September 03, 2010, Colin Cross wrote:
    > On Thu, Sep 2, 2010 at 4:09 PM, Rafael J. Wysocki <> wrote:
    > > On Friday, September 03, 2010, Colin Cross wrote:
    > >> On Thu, Sep 2, 2010 at 2:34 PM, Alan Stern <> wrote:
    > >> > On Thu, 2 Sep 2010, Colin Cross wrote:
    > >> >
    > >> >> That would work, but I still don't see why it's better. With either
    > >> >> of your changes, the power.completion variable is storing state, and
    > >> >> not just used for notification. However, the exact meaning of that
    > >> >> state is unclear, especially during the transition from an aborted
    > >> >> suspend to resume, and the state is duplicating power.status. Setting
    > >> >> it to complete in dpm_prepare is especially confusing, because at that
    > >> >> point nothing is completed, it hasn't even been started.
    > >> >
    > >> > The state being waited for varies from time to time and is only
    > >> > partially related to power.status. Instead of using a completion I
    > >> > suppose we could have used a new "transition_complete" variable
    > >> > together with a waitqueue. Would you prefer that? It's effectively
    > >> > the same thing as a completion, but without the nice packaging already
    > >> > provided by the kernel.
    > >> No, that doesn't change anything. What I'd prefer to see is a
    > >> wait_for_condition on the desired state of the parent. As is,
    > >> power.completion means one thing during suspend (the device has
    > >> started, but not finished, suspending), and a different thing during
    > >> resume (the device has not finished resuming, and may not have started
    > >> resuming). That difference is exactly what caused the bug - the
    > >> completion has to be set on init so that it is set before the device
    > >> starts suspend.
    > >
    > > Not really. The bug is there, because my analysis of the suspend error code
    > > path was wrong. Sorry about that, but it has nothing to do with the "different
    > > meaning" of the completions during suspend and resume.
    > >
    > > The completions here are simply used to enforce a specific ordering of
    > > operations, nothing more. They have no meaning beyond that.
    > The completion variable maintains state.

    So what? Locks also maintain state.

    > It has meaning whether or not you want it to. Leaving it as a completion
    > variable requires that you manage that state, which is difficult considering
    > there is no documentation and no clear idea in the code of exactly when that
    > state is set or clear.

    Please run "git show 5af84b82701a96be4b033aaa51d86c72e2ded061" and read the
    changelog. It's described in there quite clearly (I think).

    > It would be much cleaner to use a wait queue, and use
    > wait_on_condition to wait for the device to be in the desired state.

    Well, in fact that was used in one version of the patchset that introduced
    asynchronous suspend-resume, but it was rejected by Linus, because it was
    based on non-standard synchronization. The Linus' argument, that I agreed
    with, was that standard snychronization constructs, such as locks or
    completions, were guaranteed to work accross different architectures and thus
    were simply _safer_ to use than open-coded synchronization that you seem to be

    Completions simply allowed us to get the desired behavior with the least
    effort and that's why we used them.


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