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

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Date:	Tue, 15 Jun 2010 16:35:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.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 Mon, 2010-06-14 at 18:55 -0700, Andrew Morton wrote:
> > kernel/sched_clock.c:   if (cmpxchg64(&scd->clock, old_clock, clock) != old_cloc
> 
> I guess that'll flush out any stragglers.

cmpxchg64() is special, at the time i386 didn't handle the 8 byte
cmpxchg(), although we could easily make it do today.

> 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;

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