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

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Message-ID: <20160822185609.GK10153@twins.programming.kicks-ass.net>
Date:   Mon, 22 Aug 2016 20:56:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Paul McKenney <paulmck@...ux.vnet.ibm.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>,
        Steven Rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>, dvhart@...ux.intel.com,
        Frédéric Weisbecker <fweisbec@...il.com>,
        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 11:48:53AM -0700, Eric Dumazet wrote:
> On Mon, Aug 22, 2016 at 10:34 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > That does have much to say for itself, though I would prefer sizeof(void
> > *) to sizeof(unsigned long).  But would it make sense to define a mask
> > on a per-architecture basis, with the default being (sizeof(void *) - 1)?
> > Then maybe an IMPROPERLY_ALIGNED_POINTER():
> >
> >         #ifndef CONFIG_ARCH_POINTER_ALIGNMENT
> >         #define CONFIG_ARCH_POINTER_ALIGNMENT (sizeof(void *) - 1)
> >         #endif
> >
> >         #define IMPROPERLY_ALIGNED_POINTER(p) \
> >                 ((p) & CONFIG_ARCH_POINTER_ALIGNMENT)
> >
> > m68k would define ARCH_POINTER_ALIGNMENT to 1, and all other arches
> > would leave it undefined.
> >
> > Then __call_rcu() could to this:
> >
> >         WARN_ON_ONCE(IMPROPERLY_ALIGNED_POINTER(head));
> 
> Don't we have __alignof__(void *) to avoid #ifdef CONFIG_M68K and
> other new macros ?

Yes, but that 'hides' the m68k funny, while doing an explicit #ifdef has
documentation value... but I don't care too deeply.