linux-kernel - Re: [Compile Warning] 2.6.30-rc8 build

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Message-ID: <ada63fawq7y.fsf@cisco.com>
Date:	Fri, 05 Jun 2009 10:22:09 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	lkml@...eThan.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [Compile Warning] 2.6.30-rc8 build

 > To my reading of the function, I think gcc has a point:
 > 
 > drivers/serial/8250.c: In function 'serial8250_shutdown':
 > drivers/serial/8250.c:1685: warning: 'i' may be used uninitialized in this function
 > 
 > It does read as if the code might try to initialize
 > the 'lock' field of a null pointer.

The code in question is:

	static void serial_unlink_irq_chain(struct uart_8250_port *up)
	{
		struct irq_info *i;
		struct hlist_node *n;
		struct hlist_head *h;
	
		mutex_lock(&hash_mutex);
	
		h = &irq_lists[up->port.irq % NR_IRQ_HASH];
	
		hlist_for_each(n, h) {
			i = hlist_entry(n, struct irq_info, node);
			if (i->irq == up->port.irq)
				break;
		}
	
		BUG_ON(n == NULL);
		BUG_ON(i->head == NULL);
	
		if (list_empty(i->head))
			free_irq(up->port.irq, i);

and if the hlist_for_each() doesn't find a matching irq_info to put in
i, then the BUG_ON(n == NULL) will kill the system.  So there's no bug
although it is understandable that gcc can't see that.

(Not sure why you talk about "the 'lock' field of a null pointer" -- I
assume your gcc warns about the function serial8250_shutdown() because
it is inlining a function only called from a single location)

 - R.
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