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 Tue, Aug 23, 2016 at 03:45:51PM +0200, Geert Uytterhoeven wrote:
> Hi Paul,
> 
> On Tue, Aug 23, 2016 at 3:43 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Tue, Aug 23, 2016 at 08:39:18AM +0200, Geert Uytterhoeven wrote:
> >> On Mon, Aug 22, 2016 at 11:16 PM, Paul E. McKenney
> >> <paulmck@linux.vnet.ibm.com> wrote:
> >> > On Mon, Aug 22, 2016 at 10:48:57PM +0200, Geert Uytterhoeven wrote:
> >> >> On Mon, Aug 22, 2016 at 9:54 PM, Paul E. McKenney
> >> >> <paulmck@linux.vnet.ibm.com> wrote:
> >> >> > 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 *))));
> >> >>
> >> >> No, it's aligning to sizeof(void *) (4 on m68k), not __alignof__(void *).
> >> >
> >> > Right you are.  Commit 720abae3d68ae from Kirill A. Shutemov in November
> >> > 2015.
> >> >
> >> > Given that you haven't complained, I am guessing that this works for you.
> >> > If so, I can make the __call_rcu() WARN_ON() more strict.
> >> > Again, does the current state work for you?
> 
> >> Yes it does. See also your commit 1146edcbef378922 ("rcu: Loosen __call_rcu()'s
> >> rcu_head alignment constraint").
> >
> > Understood!
> >
> > But given that all architectures now provide at least four-byte alignment
> > for the rcu_head structure, isn't it now OK for me to tighten up __call_rcu()'s
> > check, for example, to this?
> >
> >         WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
> 
> Yes, I agree with that.

Very good, I have queued the following patch.

							Thanx, Paul

------------------------------------------------------------------------

commit 89d39c83d193733ed5fff1c480cd42c9de1da404
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Tue Aug 23 06:51:47 2016 -0700

    rcu: Tighted up __call_rcu() rcu_head alignment check
    
    Commit 720abae3d68ae ("rcu: force alignment on struct
    callback_head/rcu_head") forced the rcu_head (AKA callback_head)
    structure's alignment to pointer size, that is, to 4-byte boundaries on
    32-bit systems and to 8-byte boundaries on 64-bit systems.  This
    commit therefore checks for this same alignment in __call_rcu(),
    which used to check for two-byte alignment.
    
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Geert Uytterhoeven <geert@linux-m68k.org>
    Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3a8eec3ba1bd..673bcb3934a3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3122,7 +3122,9 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func,
 	unsigned long flags;
 	struct rcu_data *rdp;
 
-	WARN_ON_ONCE((unsigned long)head & 0x1); /* Misaligned rcu_head! */
+	/* Misaligned rcu_head! */
+	WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
+
 	if (debug_rcu_head_queue(head)) {
 		/* Probable double call_rcu(), so leak the callback. */
 		WRITE_ONCE(head->func, rcu_leak_callback);