using long instead of atomic_t when only set/read is required (was Re: [Bug 10030] Suspend doesn't work when SD card is inserted)

Pavel Machek
2008-02-25T11:30:43Z

Hi!

Alan thinks that `subj` is correct...

> > > > 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 always thought that reads and writes of pointers are atomic, just
> like reads and writes of longs. Is that wrong?

...but I'm not that sure. Can someone clarify?

I guess it only works as long as longs are aligned? Should it be
written down to atomic_ops.txt?
Pavel
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