Re: +workqueue-proper-error-unwinding-in-cpu-hotplug-error-path.patchadded to -mm tree

From: Akinobu Mita
Date: Tue Jul 22 2008 - 05:00:00 EST

On Tue, Jul 22, 2008 at 12:45:26PM +0400, Oleg Nesterov wrote:
> On 07/22, Andrew Morton wrote:
> >
> > From: Akinobu Mita <akinobu.mita@xxxxxxxxx>
> >
> > Add proper error unwinding in error path in CPU_UP_PREPARE notifier.
> Could you clarify?


> > --- a/kernel/workqueue.c~workqueue-proper-error-unwinding-in-cpu-hotplug-error-path
> > +++ a/kernel/workqueue.c
> > @@ -928,6 +928,15 @@ static int __devinit workqueue_cpu_callb
> > break;
> > printk(KERN_ERR "workqueue [%s] for %i failed\n",
> > wq->name, cpu);
> > +
> > + list_for_each_entry_continue_reverse(wq, &workqueues,
> > + list) {
> > + cwq = per_cpu_ptr(wq->cpu_wq, cpu);
> > + start_workqueue_thread(cwq, -1);
> > + cleanup_workqueue_thread(cwq);
> > + }
> > + cpu_clear(cpu, cpu_populated_map);
> > +
> > return NOTIFY_BAD;
> If CPU_UP_PREPARE fails, _cpu_up() sends CPU_UP_CANCELED, and afaics
> workqueue_cpu_callback() correctly cleanups cwq->thread's.

_cpu_up() does not send CPU_UP_CANCELED to the callback which has
returned NOTIFY_BAD.

The behavior was changed by this commit:

commit a0d8cdb652d35af9319a9e0fb7134de2a276c636
Author: Akinobu Mita <akinobu.mita@xxxxxxxxx>
Date: Thu Oct 18 03:05:12 2007 -0700

cpu hotplug: cpu: deliver CPU_UP_CANCELED only to NOTIFY_OKed callbacks with CPU_UP_PREPARE

The functions in a CPU notifier chain is called with CPU_UP_PREPARE event
before making the CPU online. If one of the callback returns NOTIFY_BAD, it
stops to deliver CPU_UP_PREPARE event, and CPU online operation is canceled.
Then CPU_UP_CANCELED event is delivered to the functions in a CPU notifier
chain again.

This CPU_UP_CANCELED event is delivered to the functions which have been
called with CPU_UP_PREPARE, not delivered to the functions which haven't been
called with CPU_UP_PREPARE.

The problem that makes existing cpu hotplug error handlings complex is that
the CPU_UP_CANCELED event is delivered to the function that has returned

Usually we don't expect to call destructor function against the object that
has failed to initialize. It is like:

err = register_something();
if (err) {
return err;

So it is natural to deliver CPU_UP_CANCELED event only to the functions that
have returned NOTIFY_OK with CPU_UP_PREPARE event and not to call the function
that have returned NOTIFY_BAD. This is what this patch is doing.

Otherwise, every cpu hotplug notifiler has to track whether notifiler event is
failed or not for each cpu. (drivers/base/topology.c is doing this with

Similary this patch makes same thing with CPU_DOWN_PREPARE and CPU_DOWN_FAILED

Acked-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Signed-off-by: Akinobu Mita <akinobu.mita@xxxxxxxxx>
Cc: Gautham R Shenoy <ego@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 38033db..a21f71a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -150,6 +150,7 @@ static int _cpu_down(unsigned int cpu, int tasks_frozen)
err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,
hcpu, -1, &nr_calls);
if (err == NOTIFY_BAD) {
+ nr_calls--;
__raw_notifier_call_chain(&cpu_chain, CPU_DOWN_FAILED | mod,
hcpu, nr_calls, NULL);
printk("%s: attempt to take down CPU %u failed\n",
@@ -233,6 +234,7 @@ static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen)
ret = __raw_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE | mod, hcpu,
-1, &nr_calls);
if (ret == NOTIFY_BAD) {
+ nr_calls--;
printk("%s: attempt to bring up CPU %u failed\n",
__FUNCTION__, cpu);
ret = -EINVAL;
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