Re: [PATCH tip/core/rcu 2/2] documentation: Record reason for rcu_head two-byte alignment

[Paul E. McKenney]

On Mon, Aug 22, 2016 at 06:25:53PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 22, 2016 at 08:14:43AM -0700, Paul E. McKenney wrote:
> > The __call_rcu() assertion that checks only the bottom bit of the
> > rcu_head pointer is a bit counter-intuitive in these days of ubiquitous
> > 64-bit systems.  This commit therefore records the reason for this
> > odd alignment check, namely that m68k guarantees only two-byte alignment
> > despite being a 32-bit architectures.
> 
> Would not something like:
> 
> #ifdef CONFIG_M68K
> 	/*
> 	 * m68k is weird and doesn't have naturally aligned types.
> 	 */
> 	WARN_ON_ONCE((unsigned long)head & 1);
> #else
> 	WARN_ON_ONCE((unsigned long)head & (sizeof(unsigned long) - 1));
> #endif
> 
> Be better?

That does have much to say for itself, though I would prefer sizeof(void
*) to sizeof(unsigned long).  But would it make sense to define a mask
on a per-architecture basis, with the default being (sizeof(void *) - 1)?
Then maybe an IMPROPERLY_ALIGNED_POINTER():

	#ifndef CONFIG_ARCH_POINTER_ALIGNMENT
	#define CONFIG_ARCH_POINTER_ALIGNMENT (sizeof(void *) - 1)
	#endif

	#define IMPROPERLY_ALIGNED_POINTER(p) \
		((p) & CONFIG_ARCH_POINTER_ALIGNMENT)

m68k would define ARCH_POINTER_ALIGNMENT to 1, and all other arches
would leave it undefined.

Then __call_rcu() could to this:

	WARN_ON_ONCE(IMPROPERLY_ALIGNED_POINTER(head));

Seem reasonable?

							Thanx, Paul