linux-kernel - Re: [PATCH 1/2] Add round_jiffies

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Date:	Wed, 05 Nov 2008 12:33:19 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] Add round_jiffies_up and related routines

Alan Stern wrote:
> This patch (as1158) adds round_jiffies_up() and friends.  These
> routines work like the analogous round_jiffies() functions, except
> that they will never round down.
> 
> The new routines will be useful for timeouts where we don't care
> exactly when the timer expires, provided it doesn't expire too soon.
> 
> Signed-off-by: Alan Stern <stern@...land.harvard.edu>

Heh... I have exactly the same patches but mines were named
round_up_jiffies().

> +	unsigned long j0 = jiffies;
> +
> +	barrier();	/* Prevent the compiler from aliasing j0 and jiffies */
> +	return round_jiffies_common(j + j0, cpu, false) - j0;

jiffies is volatile.  No need for explicit barrier, but this part is
necessary for correct operation as if jiffies go up by two the
calculation will wrap and the returned value will be very large.  I
think this fix deserves a separate patch and proper explanation.

Thanks.

-- 
tejun
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