On Sat, Jun 07, 2014 at 03:17:24PM +0200, Mikael Pettersson wrote:
Paul E. McKenney writes: > 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@xxxxxxxxxxx> 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...) As stated above, m68k-linux aligns to 16-bit boundaries by default, so you'd get one bit but not necessarily more. If you want more free low bits, why not attach an explicit attribute aligned to the rcu_head type declaration?
One bit should do it for the time being, but yes, if I ever need two bits, your suggestion of explicitly aligning the rcu_head type declaration sounds like a very good one. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-m68k" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
