[Compile Warning] 2.6.30-rc8 build

[Compile Warning] 2.6.30-rc8 build

[Michael S. Zick]

Group;

I can't argue with gcc on this one either:

drivers/scsi/sd.c: In function 'sd_read_capacity':
drivers/scsi/sd.c:1451: warning: comparison is always false due to limited range of data type

It reads to my eyes as if the function can never
select read_capacity_16 for very large devices.

Suggestions?

Mike

Re: [Compile Warning] 2.6.30-rc8 build

[Roland Dreier]

 > I can't argue with gcc on this one either:
 > 
 > drivers/scsi/sd.c: In function 'sd_read_capacity':
 > drivers/scsi/sd.c:1451: warning: comparison is always false due to limited range of data type
 > 
 > It reads to my eyes as if the function can never
 > select read_capacity_16 for very large devices.

The code is:

		if ((sizeof(sdkp->capacity) > 4) &&
		    (sdkp->capacity > 0xffffffffULL)) {

sdkp->capacity is a sector_t, and <linux/types.h> has:

	#ifdef CONFIG_LBD
	typedef u64 sector_t;
	typedef u64 blkcnt_t;
	#else
	typedef unsigned long sector_t;
	typedef unsigned long blkcnt_t;
	#endif

so if you don't set CONFIG_LBD on a 32-bit architecture, then you are
correct that sd.c won't ever hit the READ_CAPACITY(16) case, and the
kernel won't be able to handle large SCSI disks.

Re: [Compile Warning] 2.6.30-rc8 build

[Michael S. Zick]

On Fri June 5 2009, Roland Dreier wrote:
> 
>  > I can't argue with gcc on this one either:
>  > 
>  > drivers/scsi/sd.c: In function 'sd_read_capacity':
>  > drivers/scsi/sd.c:1451: warning: comparison is always false due to limited range of data type
>  > 
>  > It reads to my eyes as if the function can never
>  > select read_capacity_16 for very large devices.
> 
> The code is:
> 
> 		if ((sizeof(sdkp->capacity) > 4) &&
> 		    (sdkp->capacity > 0xffffffffULL)) {
> 
> sdkp->capacity is a sector_t, and <linux/types.h> has:
> 
> 	#ifdef CONFIG_LBD
> 	typedef u64 sector_t;
> 	typedef u64 blkcnt_t;
> 	#else
> 	typedef unsigned long sector_t;
> 	typedef unsigned long blkcnt_t;
> 	#endif
> 
> so if you don't set CONFIG_LBD on a 32-bit architecture, then you are
> correct that sd.c won't ever hit the READ_CAPACITY(16) case, and the
> kernel won't be able to handle large SCSI disks.
> 
> 

Perhaps then let gcc coherce the test value into a local sector_t holder
to make the warning go away (it would still be protected by the >4 thing).

Not a problem here - NetBooks don't (yet) come with that big a disk.  ;)

Thanks
Mike

[Compile Warning] 2.6.30-rc8 build

[Michael S. Zick]

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?

Mike

Re: [Compile Warning] 2.6.30-rc8 build

[Roland Dreier]

 > 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.

Re: [Compile Warning] 2.6.30-rc8 build

[Michael S. Zick]

On Fri June 5 2009, Roland Dreier wrote:
>  > 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)
>

Later in the code that gcc thought had the problem - -
where it tries to do a spinlock_init(i->lock).

Of course, I, just like gcc, did not know the machine had already died.
I'll stick an "i = something" at the top (NULL?) just to shut up gcc.

Mike 
>  - R.
> 
> 

Re: [Compile Warning] 2.6.30-rc8 build

[Roland Dreier]

 > I'll stick an "i = something" at the top (NULL?) just to shut up gcc.

you can do uninitialized_var(i) as a zero-overhead way to handle this.

Re: [Compile Warning] 2.6.30-rc8 build

[Alan Cox]

> Of course, I, just like gcc, did not know the machine had already died.
> I'll stick an "i = something" at the top (NULL?) just to shut up gcc.

And it will not get merged. Current gcc understands the code perfectly
well.

Re: [Compile Warning] 2.6.30-rc8 build

[Alan Cox]

On Fri, 5 Jun 2009 11:43:03 -0500
"Michael S. Zick" <lkml@morethan.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.

Alan

Re: [Compile Warning] 2.6.30-rc8 build

[Andrew Morton]

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

> On Fri, 5 Jun 2009 11:43:03 -0500
> "Michael S. Zick" <lkml@morethan.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().

Re: [Compile Warning] 2.6.30-rc8 build

[Alan Cox]

> 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().

Why does it care ?

Suppose the list is empty, n is loaded with NULL

That follows the BUG_ON path which expands to include a function marked
as not returning

gcc is just rather smarter than your average cc

Re: [Compile Warning] 2.6.30-rc8 build

[Andrew Morton]

On Fri, 5 Jun 2009 23:06:41 +0100
Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:

> > 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().
> 
> Why does it care ?
> 
> Suppose the list is empty, n is loaded with NULL
> 
> That follows the BUG_ON path which expands to include a function marked
> as not returning

Ah, OK.

btw, I think there are still several architectures whose BUG() isn't
correctly set up to tell gcc that it doesn't return.  Not that this is
a reason to change the serial code.