linux-kernel - Re: [RFC PATCH 17/30 v3] initialize the clock source to jiffies clock.

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Date:	Tue, 15 Jan 2008 16:14:04 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [RFC PATCH 17/30 v3] initialize the clock source to jiffies
	clock.

* Steven Rostedt (rostedt@...dmis.org) wrote:
> The latency tracer can call clocksource_read very early in bootup and
> before the clock source variable has been initialized. This results in a
> crash at boot up (even before earlyprintk is initialized). Since the
> clock->read variable points to NULL.
> 
> This patch simply initializes the clock to use clocksource_jiffies, so
> that any early user of clocksource_read will not crash.
> 

Hrm, is it sane at all to use the jiffies as a clocksource at early
boot? I thought it was updated by the timer interrupt, which is only
activated late in the boot process.

> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> Acked-by: John Stultz <johnstul@...ibm.com>
> ---
>  include/linux/clocksource.h |    3 +++
>  kernel/time/timekeeping.c   |    9 +++++++--
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> Index: linux-compile.git/include/linux/clocksource.h
> ===================================================================
> --- linux-compile.git.orig/include/linux/clocksource.h	2008-01-14 13:14:14.000000000 -0500
> +++ linux-compile.git/include/linux/clocksource.h	2008-01-14 14:57:46.000000000 -0500
> @@ -274,6 +274,9 @@ extern struct clocksource* clocksource_g
>  extern void clocksource_change_rating(struct clocksource *cs, int rating);
>  extern void clocksource_resume(void);
>  
> +/* used to initialize clock */
> +extern struct clocksource clocksource_jiffies;
> +
>  #ifdef CONFIG_GENERIC_TIME_VSYSCALL
>  extern void update_vsyscall(struct timespec *ts, struct clocksource *c);
>  extern void update_vsyscall_tz(void);
> Index: linux-compile.git/kernel/time/timekeeping.c
> ===================================================================
> --- linux-compile.git.orig/kernel/time/timekeeping.c	2008-01-14 13:14:14.000000000 -0500
> +++ linux-compile.git/kernel/time/timekeeping.c	2008-01-14 14:57:46.000000000 -0500
> @@ -53,8 +53,13 @@ static inline void update_xtime_cache(u6
>  	timespec_add_ns(&xtime_cache, nsec);
>  }
>  
> -static struct clocksource *clock; /* pointer to current clocksource */
> -
> +/*
> + * pointer to current clocksource
> + *  Just in case we use clocksource_read before we initialize
> + *  the actual clock source. Instead of calling a NULL read pointer
> + *  we return jiffies.
> + */
> +static struct clocksource *clock = &clocksource_jiffies;
>  
>  #ifdef CONFIG_GENERIC_TIME
>  /**
> 
> -- 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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