linux-kernel - Re: [PATCH 4/7] time: Update timekeeper structure using a local shadow

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Message-ID: <20120228081222.GE21106@elte.hu>
Date:	Tue, 28 Feb 2012 09:12:22 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	John Stultz <john.stultz@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH 4/7] time: Update timekeeper structure using a local
 shadow


* John Stultz <john.stultz@...aro.org> wrote:

>  static void update_wall_time(void)
>  {
>  	struct clocksource *clock;
> +	struct timekeeper tk;
>  	cycle_t offset;
>  	int shift = 0, maxshift;
>  	unsigned long flags;
> @@ -1063,10 +1064,11 @@ static void update_wall_time(void)
>  	if (unlikely(timekeeping_suspended))
>  		goto out;
>  
> -	clock = timekeeper.clock;
> +	tk = timekeeper;

'tk' is now an on-stack copy of a very non-trivial (read: large) 
structure - including locks, amongst other things. That's a 
no-no.

> +	timekeeper = tk;

You just broke lockdep here.

It's also ugly code: global data structures should almost always 
be updated in situ, instead of updating a local copy and then 
copying it back blindly ...

Thanks,

	Ingo
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