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


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