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    Subjectusing long instead of atomic_t when only set/read is required (was Re: [Bug 10030] Suspend doesn't work when SD card is inserted)
    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
    --
    (english) http://www.livejournal.com/~pavelmachek
    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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