Re: bit ops on unsigned long?

[Rusty Russell <rusty@rustcorp.com.au>]

In message <200203250245.g2P2jQa20821@vindaloo.ras.ucalgary.ca> you write:
> Rusty Russell writes:
> > Richard: 3 bugs in devfs.  Particularly note that the memset was
> > bogus.  I can't convince myself that your memcpy & memset stuff is
> > right anyway, given that you can ONLY treat them as unsigned longs
> > (ie. bit 31 will be in byte 0 or byte 3, depending on endianness).
> 
> Yes, the memset is bogus because I didn't cast the pointer to a
> char * or void *.

Yes.

> The memcpy should be fine, though. And so should
> everything else, because the bitfield array is allocated in 16 byte
> multiples.

No:

 struct major_list
 {
     spinlock_t lock;
-    __u32 bits[8];
+    unsigned long bits[256 / BITS_PER_LONG];
 };
 
 /*  Block majors already assigned:
@@ -212,7 +212,7 @@
 struct minor_list
 {
     int major;
-    __u32 bits[8];
+    unsigned long bits[256 / BITS_PER_LONG];
     struct minor_list *next;
 };

These changed are required because otherwise you try to do set_bit on
something not aligned as a long on all archs.

(Turning to the gallery) I assert: if you're going to do bitops on it,
make it a "unsigned long".

> So there should be no issues with big vs. little endian,
> since memset/memcpy operations are done in blocks of sufficient
> alignment.

I think you're right, as long as length is always a multiple of
sizeof(long).  This is not obvious from this hunk of code alone, which
is why I queried it...

    if (space->num_free < 1)
    {
	if (space->length < 16) length = 16;
	else length = space->length << 1;
	if ( ( bits = vmalloc (length) ) == NULL )
	{
	    up (&space->semaphore);
	    return -ENOMEM;
	}
	if (space->bits != NULL)
	{
	    memcpy (bits, space->bits, space->length);
	    vfree (space->bits);
	}
	space->num_free = (length - space->length) << 3;
	space->bits = bits;
	memset (bits + space->length, 0, length - space->length);
	space->length = length;
    }

Thanks!
Rusty.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.