Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33) | Date | Tue, 8 Dec 2009 21:30:14 +0100 |
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On Tuesday 08 December 2009, Alan Stern wrote: > On Tue, 8 Dec 2009, Rafael J. Wysocki wrote: > > > Suppose we use rwsem and during suspend each child uses a down_read() on a > > parent and then the parent uses down_write() on itself. What if, whatever the > > reason, the parent is a bit early and does the down_write() before one of the > > children has a chance to do the down_read()? Aren't we toast? > > > > Do we need any direct protection against that or does it just work itself out > > in a way I just don't see right now? > > 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. > > 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. 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. > > This is a little more awkward because it requires the parent to iterate > through its children.
I can live with that.
> But it does solve the off-tree dependency problem for suspends.
That's a plus, but I still think we're trying to create a barrier-alike mechanism using lock.
There's one more possibility to consider, though. What if we use a completion instead of the flag + wait queue? It surely is a standard synchronization mechanism and it seems it might work here.
Rafael
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