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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Sat, 7 Jun 2014 15:17:24 +0200
From:	Mikael Pettersson <mikpelinux@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Greg Ungerer <gerg@...inux.org>,
	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

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@...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...)

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?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/