On Sun, 24 Feb 2008, Pavel Machek wrote:

quoted text
> > > What locking protects this variable? What happens when suspending_task
> > > exits? (Hmm, that would probably be bug, anyway?)
> > 
> > It's protected by whatever existing locking scheme allows only one
> > task to start a system sleep at a time.  For example, the suspending 
> > task has to get a write lock on pm_sleep_rwsem.
> 
> And readers of suspending_task are protected by?

I added a comment about that too.

quoted text
> At the very least, you'd need rmb() before reading it and wmb() after
> writing to it, but I'm not sure if that's enough on every obscure
> architecture out there.

No, neither one is needed because of the way suspending_task is used.  

It's not necessary for a reader R to see the variable's actual value;  
all R needs to know is whether or not suspending_task is equal to R.  
Since the only process which can set suspending_task to R is R itself,
and since R will set suspending_task back to NULL before releasing the
write lock on pm_sleep_rwsem, there's never any ambiguity.

Alan Stern

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