On Sunday, 24 of February 2008, Pavel Machek wrote:
quoted text
> On Sun 2008-02-24 15:33:01, Alan Stern wrote:
> > On Sun, 24 Feb 2008, Pavel Machek wrote:
> > 
> > > > > 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.
> > 
> > > 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.
> 
> Subtle.
> 
> Very subtly wrong ;-).
> 
> imagine suspending_task == 0xabcdef01. Now task "R" with current ==
> 0xabcd0000 reads suspending_task while the other cpu is writing to it,
> and sees 0xabcd0000 (0xef01 was not yet written) -- and mistakenly
> believes that  "R" == suspending_task.
> 
> I agree it is very unlikely, and it will not happen on i386. But what
> about just using atomic_t suspending_task, and store current->pid into
> it?

I'd rather use a lock, frankly.  For example, we can require the readers to
take pm_sleep_rwsem for reading in order to access that.

Thanks,
Rafael
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