Re: 2.6.19-rc2 cpu hotplug lockdep warning: possible circular locking dependency

Neil Brown
2006-10-16T22:59:12Z

On Monday October 16, kiran@scalex86.org wrote:
> (Was: [patch] slab: Fix a cpu hotplug race condition while tuning slab cpu
> caches)
>
> On Mon, Oct 16, 2006 at 12:48:08PM -0700, Andrew Morton wrote:
> > >
> > > Not when I tested it. I just retested with lockdep on and things seemed
> > > fine on a SMP.
> >
> > Great, thanks. Please ensure that lockdep is used when testing the kernel.
> > Also preempt, DEBUG_SLAB, DEBUG_SPINLOCK_SLEEP and various other things.
> > I guess the list in Documentation/SubmitChecklist is the definitive one.
>
> Seems like cpu offline spits out lockdep warnings when actually offlining a
> CPU with CPU_HOTPLUG + CONFIG_PREEMPT + CONFIG_SLAB_DEBUG etc. Here is the
> trace I get. Note that this is plain 2.6.19-rc2 (_without_ the slab cpu
> hotplug fix).

That one is fixed by the following patch which will be in the next
-mm.

Thanks,
NeilBrown

Convert cpu hotplug notifiers to use raw_notifier instead of blocking_notifier

The use of blocking notifier by _cpu_up and _cpu_down in cpu.c
has two problem.

1/ An interaction with the workqueue notifier causes lockdep to
spit a warning.
2/ A notifier could conceivable be added or removed while _cpu_up or
_cpu_down are in process. As each notifier is called twice
(prepare then commit/abort) this could be unhealthy.

To fix to we simply take cpu_add_remove_lock while adding
or removing notifiers to/from the list.

This makes the 'blocking' usage unnecessary as all accesses to
cpu_chain are now protected by cpu_add_remove_lock. So
change "blocking" to "raw" in all relevant places.
This fixes 1.

Credit: Andrew Morton
Cc: rusty@rustcorp.com.au

Signed-off-by: Neil Brown <neilb@suse.de>

### Diffstat output
./kernel/cpu.c | 24 +++++++++++++++---------
1 file changed, 15 insertions(+), 9 deletions(-)

diff .prev/kernel/cpu.c ./kernel/cpu.c
--- .prev/kernel/cpu.c 2006-10-16 15:41:11.000000000 +1000
+++ ./kernel/cpu.c 2006-10-13 14:33:49.000000000 +1000
@@ -19,7 +19,7 @@
static DEFINE_MUTEX(cpu_add_remove_lock);
static DEFINE_MUTEX(cpu_bitmask_lock);

-static __cpuinitdata BLOCKING_NOTIFIER_HEAD(cpu_chain);
+static __cpuinitdata RAW_NOTIFIER_HEAD(cpu_chain);

/* If set, cpu_up and cpu_down will return -EBUSY and do nothing.
* Should always be manipulated under cpu_add_remove_lock
@@ -68,7 +68,11 @@ EXPORT_SYMBOL_GPL(unlock_cpu_hotplug);
/* Need to know about CPUs going up/down? */
int __cpuinit register_cpu_notifier(struct notifier_block *nb)
{
- return blocking_notifier_chain_register(&cpu_chain, nb);
+ int ret;
+ mutex_lock(&cpu_add_remove_lock);
+ ret = raw_notifier_chain_register(&cpu_chain, nb);
+ mutex_unlock(&cpu_add_remove_lock);
+ return ret;
}

#ifdef CONFIG_HOTPLUG_CPU
@@ -77,7 +81,9 @@ EXPORT_SYMBOL(register_cpu_notifier);

void unregister_cpu_notifier(struct notifier_block *nb)
{
- blocking_notifier_chain_unregister(&cpu_chain, nb);
+ mutex_lock(&cpu_add_remove_lock);
+ raw_notifier_chain_unregister(&cpu_chain, nb);
+ mutex_unlock(&cpu_add_remove_lock);
}
EXPORT_SYMBOL(unregister_cpu_notifier);

@@ -126,7 +132,7 @@ static int _cpu_down(unsigned int cpu)
if (!cpu_online(cpu))
return -EINVAL;

- err = blocking_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
+ err = raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
(void *)(long)cpu);
if (err == NOTIFY_BAD) {
printk("%s: attempt to take down CPU %u failed\n",
@@ -146,7 +152,7 @@ static int _cpu_down(unsigned int cpu)

if (IS_ERR(p)) {
/* CPU didn't die: tell everyone. Can't complain. */
- if (blocking_notifier_call_chain(&cpu_chain, CPU_DOWN_FAILED,
+ if (raw_notifier_call_chain(&cpu_chain, CPU_DOWN_FAILED,
(void *)(long)cpu) == NOTIFY_BAD)
BUG();

@@ -169,7 +175,7 @@ static int _cpu_down(unsigned int cpu)
put_cpu();

/* CPU is completely dead: tell everyone. Too late to complain. */
- if (blocking_notifier_call_chain(&cpu_chain, CPU_DEAD,
+ if (raw_notifier_call_chain(&cpu_chain, CPU_DEAD,
(void *)(long)cpu) == NOTIFY_BAD)
BUG();

@@ -206,7 +212,7 @@ static int __devinit _cpu_up(unsigned in
if (cpu_online(cpu) || !cpu_present(cpu))
return -EINVAL;

- ret = blocking_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu);
+ ret = raw_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu);
if (ret == NOTIFY_BAD) {
printk("%s: attempt to bring up CPU %u failed\n",
__FUNCTION__, cpu);
@@ -223,11 +229,11 @@ static int __devinit _cpu_up(unsigned in
BUG_ON(!cpu_online(cpu));

/* Now call notifier in preparation. */
- blocking_notifier_call_chain(&cpu_chain, CPU_ONLINE, hcpu);
+ raw_notifier_call_chain(&cpu_chain, CPU_ONLINE, hcpu);

out_notify:
if (ret != 0)
- blocking_notifier_call_chain(&cpu_chain,
+ raw_notifier_call_chain(&cpu_chain,
CPU_UP_CANCELED, hcpu);

return ret;