Re: [Xen-devel] [PATCH] xen: arm: correct use of find_next_bit

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-01-27 at 12:33 +0000, Stefano Stabellini wrote:
> On Fri, 24 Jan 2014, Ian Campbell wrote:
> > find_next_bit takes a "const unsigned long *" but forcing a cast of an
> > "uint32_t *" throws away the alignment constraints and ends up causing an
> > alignment fault on arm64 if the input happened to be 4 but not 8 byte 
> > aligned.
> 
> I am not opposed to this patch, but for the sake of clarity, isn't the
> alignment of (uint32_t*) and (const unsigned long*) the same? It should
> be 8 bytes in both cases on ARM64.

The target of a uint32_t * only needs to be 4 byte aligned on arm64,
since that is the natural alignment of a 32-bit type.

> It seems to me that the problem is not the cast to (const unsigned
> long*), but the usage of &r: maybe the tools aren't able to covert &r to
> a properly aligned pointer?

No, &r behaves exactly as expected and returns the address of r,
whatever the alignment of r is.

> Am I getting this wrong?

I think so, yes.

Ian.


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