linux-kernel - Re: [PATCH] Fix a race in pid generation that causes pids to be reused immediately.

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Message-Id: <20100615123705.232f1cf2.akpm@linux-foundation.org>
Date:	Tue, 15 Jun 2010 12:37:05 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	tytso@....edu, Salman <sqazi@...gle.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, tytso@...gle.com,
	torvalds@...ux-foundation.org, walken@...gle.com,
	Chen Liqin <liqin.chen@...plusct.com>,
	Lennox Wu <lennox.wu@...il.com>
Subject: Re: [PATCH] Fix a race in pid generation that causes pids to be
 reused immediately.

On Tue, 15 Jun 2010 16:35:52 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> > I suspect sched_clock.c might be generating fair amounts of code which
> > UP builds don't need. 
> 
> Only sched_clock_remote() and its caller, something like the below, not
> much code..
> 
> UP machines can still have utterly sucky TSC, although the
> inter-cpu-drift thing isn't much of an issue ;-)
> 
> ---
>  kernel/sched_clock.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
> index 52f1a14..7ff5b56 100644
> --- a/kernel/sched_clock.c
> +++ b/kernel/sched_clock.c
> @@ -170,6 +170,7 @@ again:
>  	return clock;
>  }
>  
> +#ifdef CONFIG_SMP
>  static u64 sched_clock_remote(struct sched_clock_data *scd)
>  {
>  	struct sched_clock_data *my_scd = this_scd();
> @@ -205,6 +206,7 @@ again:
>  
>  	return val;
>  }
> +#endif
>  
>  /*
>   * Similar to cpu_clock(), but requires local IRQs to be disabled.
> @@ -226,9 +228,11 @@ u64 sched_clock_cpu(int cpu)
>  
>  	scd = cpu_sdc(cpu);
>  
> +#ifdef CONFIG_SMP
>  	if (cpu != smp_processor_id())
>  		clock = sched_clock_remote(scd);
>  	else
> +#endif
>  		clock = sched_clock_local(scd);
>  
>  	return clock;

hm, OK, I was actually looking at sched_clock_local() at the time.  Can
clocks go backwards on UP hardware?  What breaks if we do #define
sched_clock_local sched_clock?

I've mentioned this before, but sched_clock.c is really opaque - it
would be a formidable task for anyone to get in there and work on the
code if they hadn't already been working on it for years.
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