From owner-freebsd-arch  Wed Jul 10 15:38:46 2002
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Date: Wed, 10 Jul 2002 15:37:50 -0700
From: Terry Lambert <tlambert2@mindspring.com>
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To: Archie Cobbs <archie@dellroad.org>
Cc: John Baldwin <jhb@FreeBSD.org>, davidx@viasoft.com.cn,
	freebsd-arch@FreeBSD.org, julian@elischer.org
Subject: Re: Timeout and SMP race
References: <200207102127.g6ALRl329601@arch20m.dellroad.org>
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Archie Cobbs wrote:
> Terry Lambert writes:
> > > FWIW, here is an API I've used before. This handles all race
> > > conditions and the 'other thread' question.
> >
> > I like this timer API.
> >
> > You need to add a timer_modify_interval (or something similar), so
> > that intervals can be changed without having to destroy and recreate
> > the timer, though, to avoid another potential race condition.
> 
> There's no race condition there. You can just do this to 'reset' a timer:
> 
>         timer_cancel(&foo->timer);
>         timer_start(&foo->timer, ...);
> 
> Your timer_modify_interval() would just be an optimization of this.
> 
> I forgot to mention that timer_cancel() does nothing if *handlep == NULL.


This depends on whether you mutex is local to the timer only, or
if it has other uses.

There is also an order of operation race, if you can be preempted
between the cancel and the start.

8-).

-- Terry

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