Re: [Bug 10030] Suspend doesn't work when SD card is inserted

Rafael J. Wysocki
2008-02-24T22:22:38Z

On Sunday, 24 of February 2008, Pavel Machek wrote:
> 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