Re: Lots of bugs with current->state = TASK_*INTERRUPTIBLE

Julia Lawall
2010-01-21T18:12:47Z

On Thu, 21 Jan 2010, Steven Rostedt wrote:

> On Thu, 2010-01-21 at 11:47 +0100, Julia Lawall wrote:
> > What about something like the following (drivers/macintosh/adb.c):
> >
> > add_wait_queue(&state->wait_queue, &wait);
> > current->state = TASK_INTERRUPTIBLE;
> >
> > for (;;) {
> > req = state->completed;
> > if (req != NULL)
> > state->completed = req->next;
> > else if (atomic_read(&state->n_pending) = 0)
> > ret = -EIO;
> > if (req != NULL || ret != 0)
> > break;
> >
> > if (file->f_flags & O_NONBLOCK) {
> > ret = -EAGAIN;
> > break;
> > }
> > if (signal_pending(current)) {
> > ret = -ERESTARTSYS;
> > break;
> > }
> > spin_unlock_irqrestore(&state->lock, flags);
> > schedule();
> > spin_lock_irqsave(&state->lock, flags);
> > }
> >
> > current->state = TASK_RUNNING;
> > remove_wait_queue(&state->wait_queue, &wait);
> >
> > There is a call to schedule eventually after the first current->state
> > assignment, but it is not right after.
>
> I looked at this code in a bit more detail. Seems that it does not need
> the set_current_state(), because all activities between the state of the
> task and the variables being checked (state->n_pending, et al) are under
> the state->lock.
>
> But there should be a comment stating that above the assignment of
> current->state. Something like:
>
> /*
> * No need for the set_current_state() memory barrier since
> * all checks between state and wakeups are done under the
> * state->lock.
> */
> current->state = TASK_INTERRUPTIBLE;
>
>
> But I'd rather have the author of this code write that.

As far as I can tell, state is something that is local to this driver. So
is the point that a lock is taken, or that interrupts are turned off?

julia