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

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Message-Id: <20090605145756.ed566f48.akpm@linux-foundation.org>
Date:	Fri, 5 Jun 2009 14:57:56 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	lkml@...ethan.org, linux-kernel@...r.kernel.org
Subject: Re: [Compile Warning] 2.6.30-rc8 build

On Fri, 5 Jun 2009 19:01:38 +0100
Alan Cox <alan@...rguk.ukuu.org.uk> wrote:

> On Fri, 5 Jun 2009 11:43:03 -0500
> "Michael S. Zick" <lkml@...ethan.org> wrote:
> 
> > Group,
> > 
> > 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.
> > 
> > Suggestions?
> 
> Newer gcc ? At least current gcc appears to correctly deduce the code is
> safe.

That's a gcc regression isn't it?


	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;
		}

#define hlist_for_each(pos, head) \
	for (pos = (head)->first; pos && ({ prefetch(pos->next); 1; }); \
	     pos = pos->next)

I don't think there's any way in which gcc can deduce that h->first is
non-zero on entry to that loop.  Even if it inlines
serial_unlink_irq_chain() into serial8250_shutdown().
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