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

[Neil Brown]

On Monday October 16, kiran@xxxxxxxxxxxx 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@xxxxxxxxxxxxxxx


Signed-off-by: Neil Brown <neilb@xxxxxxx>

### 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;
-
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