linux-kernel - Re: [PATCH tip/core/rcu 2/2] documentation: Record reason for rcu

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Date:   Mon, 22 Aug 2016 14:16:06 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Dumazet <edumazet@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, jiangshanlai@...il.com,
        dipankar@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Howells <dhowells@...hat.com>,
        Darren Hart <dvhart@...ux.intel.com>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        pranith kumar <bobby.prani@...il.com>
Subject: Re: [PATCH tip/core/rcu 2/2] documentation: Record reason for
 rcu_head two-byte alignment

On Mon, Aug 22, 2016 at 10:48:57PM +0200, Geert Uytterhoeven wrote:
> Hi Paul,
> 
> On Mon, Aug 22, 2016 at 9:54 PM, Paul E. McKenney
> <paulmck@...ux.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@...radead.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.

> > #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.
> 
> __alignof__(void *)  is indeed 2 on m68k, and h8300.
> 
> Note that it is 1 on crisv32!

Gah...  ((__alignof__(void *) + 1) & ~0x1), eh?

> It's 4 or 8 on anything else I have a cross-compiler for.
> 
> $ cat a.c
> unsigned x = __alignof__(void *);
> $ for i in /opt/cross/*/*/bin/*gcc; do echo +++ $i +++; $i -c -S a.c;
> cat a.s; done | less

Thank you for checking!

Again, does the current state work for you?

								Thanx, Paul