Re: [PATCH] kernel/cpu.c: Move the CPU_DYING notifiers

On Sun, Aug 31, 2008 at 07:58:49PM +0200, Manfred Spraul wrote:
When a cpu is taken offline, the CPU_DYING notifiers are called on the
dying cpu. According to <linux/notifiers.h>, the cpu should be "not
running any task, not handling interrupts, soon dead".

For the current implementation, this is not true:
- __cpu_disable can fail. If it fails, then the cpu will remain alive
and happy.
- At least on x86, __cpu_disable() briefly enables the local interrupts
to handle any outstanding interrupts.

What about moving CPU_DYING down a few lines, behind the __cpu_disable()
line?
There are only two CPU_DYING handlers in the kernel right now: one in
kvm, one in the scheduler. Both should work with the patch applied
[and: I'm not sure if either one handles a failing __cpu_disable()]

The patch survives simple offlining a cpu. kvm untested due to lack
of a test setup.

Several architectures re-enable interrupts in __cpu_disable() or in
functions called from __cpu_disable(), which happens after CPU_DYING,
if I understand correctly. :-(

Thanx, Paul

Signed-Off-By: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
---
kernel/cpu.c | 5 +++--
1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index e202a68..5b7c88f 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -199,13 +199,14 @@ static int __ref take_cpu_down(void *_param)
struct take_cpu_down_param *param = _param;
int err;

- raw_notifier_call_chain(&cpu_chain, CPU_DYING | param->mod,
- param->hcpu);
/* Ensure this CPU doesn't handle any more interrupts. */
err = __cpu_disable();
if (err < 0)
return err;

+ raw_notifier_call_chain(&cpu_chain, CPU_DYING | param->mod,
+ param->hcpu);
+
/* Force idle task to run as soon as we yield: it should
immediately notice cpu is offline and die quickly. */
sched_idle_next();
--
1.5.5.1

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