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

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Date:   Mon, 22 Aug 2016 11:48:53 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 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 ?