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?
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures)
http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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