[linux-pm] Async suspend-resume patch w/ completions (was: Re: Async suspend-resume patch w/ rwsems)

[Alan Stern]

On Sat, 12 Dec 2009, Rafael J. Wysocki wrote:

> Below is a patch I've just tested, but there's a lockdep problem in it I don't
> know how to solve.  Namely, lockdep is apparently unhappy with us not releasing
> the lock taken in device_suspend() and it complains we take it twice in a row
> (which we do, but for another device).  I need to use down_read_non_owner()
> to make it shut up and then I also need to use up_read_non_owner() in
> __device_suspend(), although there's the comment in include/linux/rwsem.h
> saying exatly this about that:
> 
> /*
>  * Take/release a lock when not the owner will release it.
>  *
>  * [ This API should be avoided as much as possible - the
>  *   proper abstraction for this case is completions. ]
>  */
> 
> (I'd like to know your opinion about that).  Yet, that's not all, because next
> it complains during resume that __device_resume() releases a lock it didn't
> acquire, which it clearly does, but that is intentional.  Unfortunately,
> there's no up_write_non_owner() ...

Hah!  I knew it!

How come lockdep didn't complain earlier?  What's different about this 
patch?  Only the nesting annotations?  Why should adding annotations 
make lockdep less happy?

Alan Stern