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

[Rafael J. Wysocki]

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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