On 8/11/05, John M. King <[email protected]> wrote:
> I know the OSS drivers are deprecated, but I'm trying to figure this out
> for my own understanding.
>
> Here's code from sound/oss/forte.c, in the write system call handler. A
> test has already been performed (under the protection of the lock) and
> the driver has decided to sleep.
>
> add_wait_queue (&channel->wait, &wait);
>
> for (;;) {
> spin_unlock_irqrestore (&chip->lock, flags);
>
> set_current_state (TASK_INTERRUPTIBLE);
> schedule();
>
> spin_lock_irqsave (&chip->lock, flags);
>
> if (channel->frag_num - channel->filled_frags)
> break;
> }
>
> remove_wait_queue (&channel->wait, &wait);
> set_current_state (TASK_RUNNING);
>
> The driver's interrupt handler calls wake_up_all(). What if an
> interrupt occurs just after the spin_unlock_irqrestore() but before
> setting TASK_INTERRUPTIBLE (and the interrupt handler does stuff that
> causes the tested conditional to be true as well)? The interrupt calls
> wake_up_all(), but then when control returns here, the process will mark
> itself TASK_INTERRUPTIBLE right away and sleep, effectively missing the
> wake_up_all().
>
> Is this a race condition? If not, can someone point out the error(s) in
> my reasoning? Please CC me as I'm not subscribed to the list.
This is broken code. You are supposed to set the state before adding
oneself to the wait-queue, if you are going to unconditionally sleep,
I believe.
So, the loop probably should be:
prepare_to_wait(&channel->wait, &wait, TASK_INTERRUPTIBLE);
for (;;) {
spin_unlock_irqrestore (&chip->lock, flags);
set_current_state (TASK_INTERRUPTIBLE); // redundant in the
first iteration
schedule();
spin_lock_irqsave (&chip->lock, flags);
if (channel->frag_num - channel->filled_frags)
break;
}
finish_wait(&channel->wait, &wait);
Thanks,
Nish
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