Re: [PATCH v3] devres: Explicitly align datai[] to 64-bit

[Peter Zijlstra]

On Mon, Jul 09, 2018 at 04:07:17PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 09, 2018 at 04:45:50PM +0300, Alexey Brodkin wrote:
> > --- a/drivers/base/devres.c
> > +++ b/drivers/base/devres.c
> > @@ -24,8 +24,12 @@ struct devres_node {
> >  
> >  struct devres {
> >  	struct devres_node		node;
> > -	/* -- 3 pointers */
> > -	unsigned long long		data[];	/* guarantee ull alignment */
> > +	/*
> > +	 * data[] must be 64 bit aligned even on 32 bit architectures
> > +	 * because it might be accessed by instructions that require
> > +	 * aligned memory arguments such as atomic64_t.
> > +	 */
> > +	u8 __aligned(8)			data[];
> >  };
> 
> Seeing that this ends up in a semi generic allocation thing, I don't
> feel this should be different from ARCH_KMALLOC_MINALIGN.

In fact, since alloc_dr() uses kmalloc() to allocate the entire thing,
it is impossible to guarantee a larger alignment than kmalloc does.