linux-kernel - [PATCH] sys_time-speedup-small-cleanup

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Message-ID: <20070626123424.GA259@tv-sign.ru>
Date:	Tue, 26 Jun 2007 16:34:24 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] sys_time-speedup-small-cleanup

on top of sys_time-speedup.patch

Ingo Molnar wrote:
>
>  asmlinkage long sys_time(time_t __user * tloc)
>  {
> -	time_t i;
> -	struct timeval tv;
> +	/*
> +	 * We read xtime.tv_sec atomically - it's updated
> +	 * atomically by update_wall_time(), so no need to
> +	 * even read-lock the xtime seqlock:
> +	 */
> +	time_t i = xtime.tv_sec;
>  
> -	do_gettimeofday(&tv);
> -	i = tv.tv_sec;
> +	smp_rmb(); /* sys_time() results are coherent */

Why do we need this barrier? My guess it is needed to prevent
the reading of xtime.tv_sec twice, yes? In that case a simple
barrier() should be enough.

Signed-off-by: Oleg Nesterov <oleg@...sign.ru>

--- t/kernel/time.c~	2007-06-26 16:28:59.000000000 +0400
+++ t/kernel/time.c	2007-06-26 16:32:09.000000000 +0400
@@ -64,7 +64,7 @@ asmlinkage long sys_time(time_t __user *
 	 */
 	time_t i = xtime.tv_sec;
 
-	smp_rmb(); /* sys_time() results are coherent */
+	barrier(); /* sys_time() results are coherent */
 
 	if (tloc) {
 		if (put_user(i, tloc))

-
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