RE: [PATCH v4 0/3] Fix find_first_zero_bit() usage

[David Laight]

From: Lorenzo Pieralisi
> Sent: 12 December 2017 15:24
> 
> On Tue, Dec 12, 2017 at 02:33:52PM +0000, David Laight wrote:
> > From: Niklas Cassel
> > > find_first_zero_bit()'s parameter 'size' is defined in bits,
> > > not in bytes.
> > >
> > > Calling find_first_zero_bit() with the wrong size unit
> > > will lead to insidious bugs.
> > >
> > > Fix all uses of find_first_zero_bit() called with
> > > sizeof() as size argument in drivers/pci.
> > ...
> >
> > Isn't all this code just using the wrong function.
> > Shouldn't they be using ffz() (or whatever it is called)
> > to find the first zero in a numeric argument rather that
> > find_first_zero_bit() which is intended for large bitmaps.
> >
> > Perhaps the type for 'large bitmaps' should be:
> > struct {
> > 	unsigned long bitmap_bits[0];
> > } bitmap;
> > rather than unsigned long[].
> 
> David,
> 
> I think you are referring to patch 3, which is a fix for the current
> find_first_zero_bit() usage. The point is, I think that
> struct pci_epc_group.function_num_map should actually be converted
> to a bitmap (and therefore using find_first_zero_bit() on it is the
> right API); patch 3 is just a fix for current code.
> 
> Unless you think patch 3 is technically wrong I would go ahead
> with the series as-is for fixes and we will refactor
> struct pci_epc_group.function_num_map usage to a proper bitmap
> for the upcoming merge window.

I may not have looked very closely at these patches, but IIRC some other
similar ones were using explicit foo |= 1 << bit to set the bit.

While technically correct (changes 4 or 8 to 32 or 64) it might be
better described as '8 * sizeof xxxx'.
Then the code is correct regardless of the bitmap size (unless smaller
than a long on (probably) BE systems).

	David