linux-kernel - Re: [PATCH] jiffies.h: Declare jiffies and jiffies_64 with ____cacheline_aligned_in

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Date:   Thu, 23 Mar 2017 13:58:13 -0700
From:   Grant Grundler <grundler@...omium.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     "Jason A . Donenfeld" <Jason@...c4.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Grant Grundler <grundler@...omium.org>,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>
Subject: Re: [PATCH] jiffies.h: Declare jiffies and jiffies_64 with ____cacheline_aligned_in_smp

Ping? Any feedback on this patch?
Is this in someone's queue for review?

thanks,
grant

On Tue, Mar 14, 2017 at 12:40 PM, Matthias Kaehlcke <mka@...omium.org> wrote:
> jiffies_64 is defined in kernel/time/timer.c with ____cacheline_aligned_in_smp,
> however this macro is not part of the declaration of jiffies and jiffies_64
> in jiffies.h.
>
> As a result clang generates the following warning:
>
> kernel/time/timer.c:57:26: error: section does not match previous declaration [-Werror,-Wsection]
> __visible u64 jiffies_64 __cacheline_aligned_in_smp = INITIAL_JIFFIES;
>                          ^
> include/linux/cache.h:39:36: note: expanded from macro '__cacheline_aligned_in_smp'
>                                    ^
> include/linux/cache.h:34:4: note: expanded from macro '__cacheline_aligned'
>                  __section__(".data..cacheline_aligned")))
>                  ^
> include/linux/jiffies.h:77:12: note: previous attribute is here
> extern u64 __jiffy_data jiffies_64;
>            ^
> include/linux/jiffies.h:70:38: note: expanded from macro '__jiffy_data'
>
> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> ---
>  include/linux/jiffies.h | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
> index 589d14e970ad..175873a814fd 100644
> --- a/include/linux/jiffies.h
> +++ b/include/linux/jiffies.h
> @@ -1,6 +1,7 @@
>  #ifndef _LINUX_JIFFIES_H
>  #define _LINUX_JIFFIES_H
>
> +#include <linux/cache.h>
>  #include <linux/math64.h>
>  #include <linux/kernel.h>
>  #include <linux/types.h>
> @@ -63,19 +64,13 @@ extern int register_refined_jiffies(long clock_tick_rate);
>  /* TICK_USEC is the time between ticks in usec assuming fake USER_HZ */
>  #define TICK_USEC ((1000000UL + USER_HZ/2) / USER_HZ)
>
> -/* some arch's have a small-data section that can be accessed register-relative
> - * but that can only take up to, say, 4-byte variables. jiffies being part of
> - * an 8-byte variable may not be correctly accessed unless we force the issue
> - */
> -#define __jiffy_data  __attribute__((section(".data")))
> -
>  /*
>   * The 64-bit value is not atomic - you MUST NOT read it
>   * without sampling the sequence number in jiffies_lock.
>   * get_jiffies_64() will do this for you as appropriate.
>   */
> -extern u64 __jiffy_data jiffies_64;
> -extern unsigned long volatile __jiffy_data jiffies;
> +extern u64 __cacheline_aligned_in_smp jiffies_64;
> +extern unsigned long volatile __cacheline_aligned_in_smp jiffies;
>
>  #if (BITS_PER_LONG < 64)
>  u64 get_jiffies_64(void);
> --
> 2.12.0.367.g23dc2f6d3c-goog
>