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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Aug 22, 2016 at 03:18:54PM -0400, Steven Rostedt wrote:
> On Mon, 22 Aug 2016 20:56:09 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > > Don't we have __alignof__(void *) to avoid #ifdef CONFIG_M68K and
> > > other new macros ?  

Hmmm...  Does __alignof__(void *) give two-byte alignment on m68k,
allowing something like this?  Heh!!!  It is already there.  ;-)

struct callback_head {
	struct callback_head *next;
	void (*func)(struct callback_head *head);
} __attribute__((aligned(sizeof(void *))));
#define rcu_head callback_head

If so, that does sound quite attractive!  Might need the WARN_ON()
anyway, to flag wild pointers if nothing else.

Adding Geert on CC for his thoughts.

> > Yes, but that 'hides' the m68k funny, while doing an explicit #ifdef has
> > documentation value... but I don't care too deeply.

Well, if I need the WARN_ON() anyway, perhaps we get both.

> I'd recommend keeping the #ifdef, and then if another architecture
> comes along that is as weird as m68k, we can use the generic
> __alignof__(void *). Maybe even add that in the comment, so when/if
> that arch is created, people will know how to fix it more generically.

Maybe __call_rcu() can do something like this?

	WARN_ON_ONCE((unsigned long)head & (__alignof__(struct rcu_head) - 1));

Except that RCU needs at least two-byte alignment, so something like this?

	WARN_ON_ONCE((unsigned long)head &
		     ((__alignof__(struct rcu_head) - 1) | 0x1));

That way, some future architecture that doesn't believe in any alignment
at all will be properly informed of RCU's needs in this area.

							Thanx, Paul