On Mon, Aug 22, 2016 at 03:18:54PM -0400, Steven Rostedt wrote:
> On Mon, 22 Aug 2016 20:56:09 +0200
> Peter Zijlstra <pet...@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
