powerpc/pmac: Don't add_timer() twice

If the interrupt and the timeout happen roughly at the same
time, we can get into a situation where the timer function
is run while the interrupt has already been processed. In
this case, the timer function might end up doing an add_timer
on an already pending timer, causing a BUG_ON() to trigger.

Instead, just skip the whole timeout operation if we see that
the timer is pending. The spinlock ensures that the only way
that happens is if we already started a new operation and thus
the timeout can be ignored.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/powermac/low_i2c.c

@@ -366,11 +366,20 @@ static void kw_i2c_timeout(unsigned long data)
 	unsigned long flags;
 
 	spin_lock_irqsave(&host->lock, flags);
+
+	/*
+	 * If the timer is pending, that means we raced with the
+	 * irq, in which case we just return
+	 */
+	if (timer_pending(&host->timeout_timer))
+		goto skip;
+
 	kw_i2c_handle_interrupt(host, kw_read_reg(reg_isr));
 	if (host->state != state_idle) {
 		host->timeout_timer.expires = jiffies + KW_POLL_TIMEOUT;
 		add_timer(&host->timeout_timer);
 	}
+ skip:
 	spin_unlock_irqrestore(&host->lock, flags);
 }