linux-kernel - Re: rcu alignment warning tripping on m68k

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Date:	Fri, 6 Jun 2014 11:46:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Greg Ungerer <gerg@...inux.org>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	Linux/m68k <linux-m68k@...r.kernel.org>
Subject: Re: rcu alignment warning tripping on m68k

On Fri, May 30, 2014 at 11:29:41AM +1000, Greg Ungerer wrote:
> On 29/05/14 23:11, One Thousand Gnomes wrote:
> > On Thu, 29 May 2014 12:08:32 +1000
> > Greg Ungerer <gerg@...inux.org> wrote:
> > 
> >> Hi All,
> >>
> >> Inside kernel/rcy/tree.c in __call_rcu() it does an alignment check on
> >> the head pointer passed in. This trips on m68k systems, because they only
> >> need alignment of 32bit quantities to 16bit boundaries.
> > 
> > __alignof perhaps ?
> 
> That might do. Change then becomes something like:
> 
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2467,7 +2467,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_
>         unsigned long flags;
>         struct rcu_data *rdp;
> 
> -       WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
> +       WARN_ON_ONCE((unsigned long)head & (__alignof__(head) - 1)); /* Misaligned rcu_head! */

Hmmm...  The purpose of the check is to reserve the low-order bits to
allow RCU to classify callbacks as being time-critical or not.  RCU
can probably live with a single bit, but if there is some architecture
out there that simply refuses to do alignment, I need to know about it.

(See "git show 0bb7b59d6e2b8" for more info.)

So how about this instead?

 -       WARN_ON_ONCE((unsigned long)head & 0x1); /* Misaligned rcu_head! */

(Trying to remember if I have seen Linux kernel code that uses both
the lower bits...)

							Thanx, Paul

>         if (debug_rcu_head_queue(head)) {
>                 /* Probable double call_rcu(), so leak the callback. */
>                 ACCESS_ONCE(head->func) = rcu_leak_callback;
> 
> Thanks
> Greg
> 
> 
> --
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