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

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Date:	Wed, 5 Nov 2008 11:20:06 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tejun Heo <tj@...nel.org>
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

On Wed, 5 Nov 2008, Tejun Heo wrote:

> 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().

To an American, "round_up_jiffies" sounds like something a cowboy might 
do.  :-)

I haven't bothered to look throughout the kernel to see where 
round_jiffies_up() could be used.  Have you done this?

> > +	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,

I didn't realize that.  Good, it makes things easier.

> 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.

How about if I remove the barrier() call?  Should this new code still
go in a separate patch?

Alan Stern

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