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Subject: PATCH: Fix race in cpu_down (hotplug cpu)
From: Nigel Cunningham <ncunningham@cyclades.com>
Reply-To: ncunningham@cyclades.com
To: Andrew Morton <akpm@osdl.org>, Linus Torvalds <torvalds@osdl.org>,
       Zwane Mwaikambo <zwane@arm.linux.org.uk>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Organization: Cyclades
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Date: Mon, 19 Sep 2005 13:15:36 +1000
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Hi.

There is a race condition in taking down a cpu (kernel/cpu.c::cpu_down).
A cpu can already be idling when we clear its online flag, and we do not
force the idle task to reschedule. This results in __cpu_die timing out.
A simple fix is to force the idle task on the cpu going to reschedule.

Without the patch below, Suspend2 get into a deadlock at resume time
when this issue occurs. I could not complete 20 cycles without seeing
the issue. With the patch below, I have completed 75 cycles on the trot
without problems.

Please apply.

Signed-off-by: Nigel Cunningham <ncunningham@cyclades.com>

diff -ruNp 9910-hotplug-cpu-race.patch-old/kernel/cpu.c 9910-hotplug-cpu-race.patch-new/kernel/cpu.c
--- 9910-hotplug-cpu-race.patch-old/kernel/cpu.c	2005-08-29 10:29:58.000000000 +1000
+++ 9910-hotplug-cpu-race.patch-new/kernel/cpu.c	2005-09-19 12:15:08.000000000 +1000
@@ -126,6 +126,9 @@ int cpu_down(unsigned int cpu)
 	while (!idle_cpu(cpu))
 		yield();
 
+	/* CPU may have idled before we set its offline flag. */
+	set_tsk_need_resched(idle_task(cpu));
+
 	/* This actually kills the CPU. */
 	__cpu_die(cpu);