On Mon, 25 Jan 1999, Andrea Arcangeli wrote:

> Here the fix for your latest code (that I merged last night).

Thanks.

> Please Tim tell me if you can reproduce the weirness also with this patch
> applyed.

I can still reproduce it.

The reason it happens is that semaphores are now recursive.  Here is the
loop:

  request_irq (..., func_that_ups_semaphore);

  tell_hardware_to_generate_irq_when_ready ();

false_alarm:

  set_a_1s_timer;

  down_interruptible (semaphore);

  if (!hardware_ready ())
    goto false_alarm;

As soon as a false alarm occurs, we already own the semaphore, and so we
spin forever.  Before semaphores went recursive, this wasn't the case.

I'm using semaphores rather than (say) waitqueues because I don't want to
lose any interrupts through things like this:

void irq_handler (...)
{
	if (!waitqueue_active(...))
		/* Missed an interrupt. */
		return;

	wake_up (...);
}

If there's another atomic primitive I should be using, please let me know,
but I kind of thought that this is what semaphores were for.

By the way, there really should be a "if (need_resched) schedule();" in
that loop, so that a confused parallel port that keeps sending interrupts
for no reason doesn't bring the system to a halt.  But that's another
story.

Tim.
*/


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