timer/hrtimer & cpu hotplug lockdep complaints

lockdep tells us that we have a possible circular locking dependency. The output below is incomplete since our console driver deadlocked on del_timer()...

    <4>Processor 4 spun down
    <4>
    <4>=======================================================
    <4>[ INFO: possible circular locking dependency detected ]
    <4>2.6.20-23.x.20070222-s390xdebug #1
    <4>-------------------------------------------------------
    <4>bash/1447 is trying to acquire lock:
    <4> (base_lock_keys + cpu#5){++..}, at: [<000000000013b686>] timer_cpu_notify+0xd2/0x3a4
    <4>
    <4>but task is already holding lock:
    <4> (base_lock_keys + cpu#7){++..}, at: [<000000000013b67c>] timer_cpu_notify+0xc8/0x3a4
    <4>
    <4>which lock already depends on the new lock.
    <4>
    <4>
    <4>the existing dependency chain (in reverse order) is:
    <4>
    <4>-> #1 (base_lock_keys + cpu#7){++..}:
    <4>       [<0000000000158380>] __lock_acquire+0xe40/0xf78
    <4>       [<000000000015854a>] lock_acquire+0x92/0xb8
    <4>       [<0000000000441eea>] _spin_lock+0x52/0x6c
    <4>       [<000000000013b686>] timer_cpu_notify+0xd2/0x3a4
    <4>       [<0000000000443c44>] notifier_call_chain+0x64/0x88
    <4>       [<000000000014200a>] raw_notifier_call_chain+0x26/0x38
    <4>       [<000000000015deee>] cpu_down+0x25e/0x350
    <4>       [<00000000002cb650>] store_online+0x64/0xb8

The problem in this case is the cpu hotplug code in kernel/timer.c:

	migrate_timers(...)
	...
	old_base = per_cpu(tvec_bases, cpu);
	new_base = get_cpu_var(tvec_bases);

	local_irq_disable();
	spin_lock(&new_base->lock);
	spin_lock(&old_base->lock);

takes per-cpu locks once in A-B order and once in B-A order, depending on from which cpu another one gets killed. This should be ok, since migrate_timers() can only be active on one cpu and therefore no deadlock should occur...
The same problem is present in kernel/hrtimer.c btw.
Any idea how to fix this? Just adding a lockdep_off/on between the two spin_lock()s will fix this, but I don't like it.

Btw.: this is the call trace that lead to the deadlock when printing the lockdep message. Looks like it is a bad idea if the console driver needs to take a lock that lockdep wants to complain about. But I don't think this needs to be fixed.

0 _raw_spin_lock+326 [0x2a90ee]
1 _spin_lock_irqsave+108 [0x442198]
2 lock_timer_base+64 [0x13c758]
3 del_timer+84 [0x13cfc4]
4 raw3215_write+610 [0x340fb6]
5 con3215_write+96 [0x3410a4]
6 __call_console_drivers+194 [0x12d95a]
7 _call_console_drivers+142 [0x12da06]
8 release_console_sem+460 [0x12dfc4]
9 vprintk+546 [0x12e922]
10 printk+82 [0x12da9e]
11 __print_symbol+220 [0x162f40]
12 print_stack_trace+136 [0x152664]
13 print_circular_bug_entry+104 [0x154f84]
14 print_circular_bug_header+260 [0x1560fc]
15 check_noncircular+354 [0x15752e]
16 __lock_acquire+2970 [0x1580da]
17 lock_acquire+146 [0x15854a]
18 _spin_lock+82 [0x441eea]
19 timer_cpu_notify+210 [0x13b686]
20 notifier_call_chain+100 [0x443c44]
21 raw_notifier_call_chain+38 [0x14200a]
22 cpu_down+606 [0x15deee]
23 store_online+100 [0x2cb650]
24 sysdev_store+72 [0x2c56d0]
25 sysfs_write_file+286 [0x20ea52]
26 vfs_write+210 [0x1ad74e]
27 sys_write+86 [0x1adf7a]
28 sysc_noemu+16 [0x111ad0]

Doing something like this on a two cpu system

# echo 0 > /sys/devices/system/cpu/cpu0/online 
# echo 1 > /sys/devices/system/cpu/cpu0/online 
# echo 0 > /sys/devices/system/cpu/cpu1/online 

will give me this:

=======================================================
[ INFO: possible circular locking dependency detected ] 2.6.21-rc2-g562aa1d4-dirty #7
-------------------------------------------------------
bash/1282 is trying to acquire lock:
(&cpu_base->lock_key){.+..}, at: [<000000000005f17e>] hrtimer_cpu_notify+0xc6/0x240

but task is already holding lock:
(&cpu_base->lock_key#2){.+..}, at: [<000000000005f174>] hrtimer_cpu_notify+0xbc/0x240

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&cpu_base->lock_key#2){.+..}:

       [<000000000006585e>] __lock_acquire+0x8d6/0x1184
       [<000000000006661c>] lock_acquire+0x90/0xc0
       [<000000000036c70a>] _spin_lock+0x4e/0x68
       [<000000000005f17e>] hrtimer_cpu_notify+0xc6/0x240
       [<000000000036e544>] notifier_call_chain+0x54/0x78
       [<00000000000508ba>] raw_notifier_call_chain+0x26/0x34
       [<000000000006d4b2>] cpu_down+0x276/0x36c
       [<00000000001c5764>] store_online+0x60/0xc0
       [<00000000001bf874>] sysdev_store+0x40/0x58
       [<000000000010aa3a>] sysfs_write_file+0x12e/0x1d0
       [<00000000000b0584>] vfs_write+0xa8/0x15c
       [<00000000000b0716>] sys_write+0x56/0x88
       [<000000000002275c>] sysc_noemu+0x10/0x16
       [<0000020000118da8>] 0x20000118da8

-> #0 (&cpu_base->lock_key){.+..}:

       [<00000000000656b8>] __lock_acquire+0x730/0x1184
       [<000000000006661c>] lock_acquire+0x90/0xc0
       [<000000000036c70a>] _spin_lock+0x4e/0x68
       [<000000000005f17e>] hrtimer_cpu_notify+0xc6/0x240
       [<000000000036e544>] notifier_call_chain+0x54/0x78
       [<00000000000508ba>] raw_notifier_call_chain+0x26/0x34
       [<000000000006d4b2>] cpu_down+0x276/0x36c
       [<00000000001c5764>] store_online+0x60/0xc0
       [<00000000001bf874>] sysdev_store+0x40/0x58
       [<000000000010aa3a>] sysfs_write_file+0x12e/0x1d0
       [<00000000000b0584>] vfs_write+0xa8/0x15c
       [<00000000000b0716>] sys_write+0x56/0x88
       [<000000000002275c>] sysc_noemu+0x10/0x16
       [<0000020000118da8>] 0x20000118da8

other info that might help us debug this:

4 locks held by bash/1282:

 #0:  (cpu_add_remove_lock){--..}, at: [<000000000036a472>] mutex_lock+0x3e/0x4c
 #1:  (cache_chain_mutex){--..}, at: [<000000000036a472>] mutex_lock+0x3e/0x4c
 #2:  (workqueue_mutex){--..}, at: [<000000000036a472>] mutex_lock+0x3e/0x4c
 #3:  (&cpu_base->lock_key#2){.+..}, at: [<000000000005f174>] hrtimer_cpu_notify+0xbc/0x240

stack backtrace:
4141e8b900000000 4141e8b900000000 3ff2c76e1deacc92 0000000000000000 
       0000000000000000 0000000000420498 0000000000420498 000000000001595e 
       0000000000000000 0000000000000004 0000000000000000 00000000004fadf0 
       0000000000000000 0000336e0000000d 000000000ce0f970 000000000ce0f9e8 
       0000000000375c30 000000000001595e 000000000ce0f970 000000000ce0f9c0 
Call Trace:
([<000000000001588a>] show_trace+0x92/0xe8)
 [<0000000000015980>] show_stack+0xa0/0xd0
 [<00000000000159de>] dump_stack+0x2e/0x3c
 [<0000000000062cdc>] print_circular_bug_tail+0xa0/0xa4
 [<00000000000656b8>] __lock_acquire+0x730/0x1184
 [<000000000006661c>] lock_acquire+0x90/0xc0
 [<000000000036c70a>] _spin_lock+0x4e/0x68
 [<000000000005f17e>] hrtimer_cpu_notify+0xc6/0x240
 [<000000000036e544>] notifier_call_chain+0x54/0x78
 [<00000000000508ba>] raw_notifier_call_chain+0x26/0x34
 [<000000000006d4b2>] cpu_down+0x276/0x36c
 [<00000000001c5764>] store_online+0x60/0xc0
 [<00000000001bf874>] sysdev_store+0x40/0x58
 [<000000000010aa3a>] sysfs_write_file+0x12e/0x1d0
 [<00000000000b0584>] vfs_write+0xa8/0x15c
 [<00000000000b0716>] sys_write+0x56/0x88
 [<000000000002275c>] sysc_noemu+0x10/0x16
 [<0000020000118da8>] 0x20000118da8

4 locks held by bash/1282:

 #0:  (cpu_add_remove_lock){--..}, at: [<000000000036a472>] mutex_lock+0x3e/0x4c
 #1:  (cache_chain_mutex){--..}, at: [<000000000036a472>] mutex_lock+0x3e/0x4c
 #2:  (workqueue_mutex){--..}, at: [<000000000036a472>] mutex_lock+0x3e/0x4c
 #3:  (&cpu_base->lock_key#2){.+..}, at: [<000000000005f174>] hrtimer_cpu_notify+0xbc/0x240

This happens because we have the following code in kernel/hrtimer.c:

migrate_hrtimers(int cpu)
[...]
old_base = &per_cpu(hrtimer_bases, cpu);
new_base = &get_cpu_var(hrtimer_bases);
[...]
spin_lock(&new_base->lock);
spin_lock(&old_base->lock);

Which means the spinlocks are taken in an order which depends on which cpu gets shut down from which other cpu. Therefore lockdep complains that there might be an ABBA deadlock. Since migrate_hrtimers() gets only called on cpu hotplug it's safe to assume that it isn't executed concurrently on a different cpu and therefore the locking should be ok.

The same problem exists in kernel/timer.c: migrate_timers(). As an added bonus we get there a locked up system on s390, since the lock that lockdep want's to complain about via printk is also needed by our 3215 console driver (calls del_timer). So we end up in a deadlock.

As pointed out by Christian Borntraeger one possible solution to avoid the locking order complaints would be to make sure that the locks are always taken in the same order. E.g. by taking the lock of the cpu with the lower number first. AFIACS this should be safe and that is what this patch does.

---
 kernel/hrtimer.c |   17 ++++++++++++-----
 kernel/timer.c   |   16 ++++++++++++----
 2 files changed, 24 insertions(+), 9 deletions(-)

Index: linux-2.6/kernel/hrtimer.c
===================================================================
@@ -1346,6 +1346,7 @@ static void migrate_hrtimer_list(struct 
 static void migrate_hrtimers(int cpu)
 {
 	struct hrtimer_cpu_base *old_base, *new_base;
+	spinlock_t *lock1, *lock2;
 	int i;
 
 	BUG_ON(cpu_online(cpu));

@@ -1355,16 +1356,22 @@ static void migrate_hrtimers(int cpu)
 	tick_cancel_sched_timer(cpu);
 
 	local_irq_disable();
-
-	spin_lock(&new_base->lock);
-	spin_lock(&old_base->lock);
+	/*
+	 * If we take a lock from a different cpu, make sure we have always
+	 * the same locking order. That is the lock that belongs to the cpu
+	 * with the lowest number is taken first.
+	 */
+	lock1 = smp_processor_id() < cpu ? &new_base->lock : &old_base->lock;
+	lock2 = smp_processor_id() < cpu ? &old_base->lock : &new_base->lock;
+	spin_lock(lock1);
+	spin_lock(lock2);
 
 	for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
 		migrate_hrtimer_list(&old_base->clock_base[i],
 				     &new_base->clock_base[i]);
 	}
-	spin_unlock(&old_base->lock);
-	spin_unlock(&new_base->lock);
+	spin_unlock(lock2);
+	spin_unlock(lock1);
 
 	local_irq_enable();
 	put_cpu_var(hrtimer_bases);

Index: linux-2.6/kernel/timer.c
===================================================================
@@ -1644,6 +1644,7 @@ static void __devinit migrate_timers(int
 {
 	tvec_base_t *old_base;
 	tvec_base_t *new_base;
+	spinlock_t *lock1, *lock2;
 	int i;
 
 	BUG_ON(cpu_online(cpu));

@@ -1651,8 +1652,15 @@ static void __devinit migrate_timers(int
 	new_base = get_cpu_var(tvec_bases);
 
 	local_irq_disable();
-	spin_lock(&new_base->lock);
-	spin_lock(&old_base->lock);
+	/*
+	 * If we take a lock from a different cpu, make sure we have always
+	 * the same locking order. That is the lock that belongs to the cpu
+	 * with the lowest number is taken first.
+	 */
+	lock1 = smp_processor_id() < cpu ? &new_base->lock : &old_base->lock;
+	lock2 = smp_processor_id() < cpu ? &old_base->lock : &new_base->lock;
+	spin_lock(lock1);
+	spin_lock(lock2);
 
 	BUG_ON(old_base->running_timer);
 

@@ -1665,8 +1673,8 @@ static void __devinit migrate_timers(int
 		migrate_timer_list(new_base, old_base->tv5.vec + i);
 	}
 
-	spin_unlock(&old_base->lock);
-	spin_unlock(&new_base->lock);
+	spin_unlock(lock2);
+	spin_unlock(lock1);
 	local_irq_enable();
 	put_cpu_var(tvec_bases);
 }

Something like the stuff below? Exploits the knowledge that the two tvec_base_t's are in a per_cpu array. Otherwise I would end up passing a lot of redundant stuff. But still I think that isn't a good solution but rather a hack...?
I'd go for the patch above.

---

Index: linux-2.6/kernel/timer.c
===================================================================
@@ -1640,6 +1640,28 @@ static void migrate_timer_list(tvec_base
 	}
 }
 
+static void __devinit double_tvec_lock(tvec_base_t *base1, tvec_base_t *base2)
+{
+	if (base1 < base2) {
+		spin_lock(&base1->lock);
+		spin_lock(&base2->lock);
+	} else {
+		spin_lock(&base2->lock);
+		spin_lock(&base1->lock);
+	}
+}
+
+static void __devinit double_tvec_unlock(tvec_base_t *base1, tvec_base_t *base2)
+{
+	if (base1 < base2) {
+		spin_unlock(&base1->lock);
+		spin_unlock(&base2->lock);
+	} else {
+		spin_unlock(&base2->lock);
+		spin_unlock(&base1->lock);
+	}
+}
+
 static void __devinit migrate_timers(int cpu)
 {
 	tvec_base_t *old_base;

@@ -1651,8 +1673,7 @@ static void __devinit migrate_timers(int
 	new_base = get_cpu_var(tvec_bases);
 
 	local_irq_disable();
-	spin_lock(&new_base->lock);
-	spin_lock(&old_base->lock);
+	double_tvec_lock(new_base, old_base);
 
 	BUG_ON(old_base->running_timer);
 

@@ -1665,8 +1686,7 @@ static void __devinit migrate_timers(int
 		migrate_timer_list(new_base, old_base->tv5.vec + i);
 	}
 
-	spin_unlock(&old_base->lock);
-	spin_unlock(&new_base->lock);
+	double_tvec_unlock(new_base, old_base);
 	local_irq_enable();
 	put_cpu_var(tvec_bases);
 }

Yeah, it'd be nicer to pass in the CPU number(s), use that to make the ordering decision. Perhaps (smp_processor_id() - cpu).

From: Heiko Carstens <[email protected]>

Doing something like this on a two cpu system

# echo 0 > /sys/devices/system/cpu/cpu0/online 
# echo 1 > /sys/devices/system/cpu/cpu0/online 
# echo 0 > /sys/devices/system/cpu/cpu1/online 

will give me this:

=======================================================
[ INFO: possible circular locking dependency detected ] 2.6.21-rc2-g562aa1d4-dirty #7
-------------------------------------------------------
bash/1282 is trying to acquire lock:
(&cpu_base->lock_key){.+..}, at: [<000000000005f17e>] hrtimer_cpu_notify+0xc6/0x240

but task is already holding lock:
(&cpu_base->lock_key#2){.+..}, at: [<000000000005f174>] hrtimer_cpu_notify+0xbc/0x240

which lock already depends on the new lock.

This happens because we have the following code in kernel/hrtimer.c:

migrate_hrtimers(int cpu)
[...]
old_base = &per_cpu(hrtimer_bases, cpu);
new_base = &get_cpu_var(hrtimer_bases);
[...]
spin_lock(&new_base->lock);
spin_lock(&old_base->lock);

Which means the spinlocks are taken in an order which depends on which cpu gets shut down from which other cpu. Therefore lockdep complains that there might be an ABBA deadlock. Since migrate_hrtimers() gets only called on cpu hotplug it's safe to assume that it isn't executed concurrently on a different cpu and therefore the locking should be ok.

The same problem exists in kernel/timer.c: migrate_timers().

As pointed out by Christian Borntraeger one possible solution to avoid the locking order complaints would be to make sure that the locks are always taken in the same order. E.g. by taking the lock of the cpu with the lower number first. AFIACS this should be safe and that is what this patch does.

---
 kernel/hrtimer.c |   39 ++++++++++++++++++++++++++++++++++-----
 kernel/timer.c   |   38 ++++++++++++++++++++++++++++++++++----
 2 files changed, 68 insertions(+), 9 deletions(-)

Index: linux-2.6/kernel/hrtimer.c
===================================================================
@@ -1343,6 +1343,38 @@ static void migrate_hrtimer_list(struct 
 	}
 }
 
+/*
+ * double_hrtimer_lock/unlock are used to ensure that on cpu hotplug the
+ * per cpu timer locks are always taken in the same order.
+ */
+static void double_hrtimer_lock(struct hrtimer_cpu_base *base1,
+				struct hrtimer_cpu_base *base2, int ind)
+	__acquires(base1->lock)
+	__acquires(base2->lock)
+{
+	if (ind < 0) {
+		spin_lock(&base1->lock);
+		spin_lock(&base2->lock);
+	} else {
+		spin_lock(&base2->lock);
+		spin_lock(&base1->lock);
+	}
+}
+
+static void double_hrtimer_unlock(struct hrtimer_cpu_base *base1,
+				  struct hrtimer_cpu_base *base2, int ind)
+	__releases(base1->lock)
+	__releases(base2->lock)
+{
+	if (ind < 0) {
+		spin_unlock(&base2->lock);
+		spin_unlock(&base1->lock);
+	} else {
+		spin_unlock(&base1->lock);
+		spin_unlock(&base2->lock);
+	}
+}
+
 static void migrate_hrtimers(int cpu)
 {
 	struct hrtimer_cpu_base *old_base, *new_base;

@@ -1355,17 +1387,14 @@ static void migrate_hrtimers(int cpu)
 	tick_cancel_sched_timer(cpu);
 
 	local_irq_disable();
-
-	spin_lock(&new_base->lock);
-	spin_lock(&old_base->lock);
+	double_hrtimer_lock(new_base, old_base, smp_processor_id() - cpu);
 
 	for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
 		migrate_hrtimer_list(&old_base->clock_base[i],
 				     &new_base->clock_base[i]);
 	}
-	spin_unlock(&old_base->lock);
-	spin_unlock(&new_base->lock);
 
+	double_hrtimer_unlock(new_base, old_base, smp_processor_id() - cpu);
 	local_irq_enable();
 	put_cpu_var(hrtimer_bases);
 }

Index: linux-2.6/kernel/timer.c
===================================================================
@@ -1640,6 +1640,38 @@ static void migrate_timer_list(tvec_base
 	}
 }
 
+/*
+ * double_timer_lock/unlock are used to ensure that on cpu hotplug the
+ * per cpu timer locks are always taken in the same order.
+ */
+static void __devinit double_timer_lock(tvec_base_t *base1,
+					tvec_base_t *base2, int ind)
+	__acquires(base1->lock)
+	__acquires(base2->lock)
+{
+	if (ind < 0) {
+		spin_lock(&base1->lock);
+		spin_lock(&base2->lock);
+	} else {
+		spin_lock(&base2->lock);
+		spin_lock(&base1->lock);
+	}
+}
+
+static void __devinit double_timer_unlock(tvec_base_t *base1,
+					  tvec_base_t *base2, int ind)
+	__releases(base1->lock)
+	__releases(base2->lock)
+{
+	if (ind < 0) {
+		spin_unlock(&base2->lock);
+		spin_unlock(&base1->lock);
+	} else {
+		spin_unlock(&base1->lock);
+		spin_unlock(&base2->lock);
+	}
+}
+
 static void __devinit migrate_timers(int cpu)
 {
 	tvec_base_t *old_base;

@@ -1651,8 +1683,7 @@ static void __devinit migrate_timers(int
 	new_base = get_cpu_var(tvec_bases);
 
 	local_irq_disable();
-	spin_lock(&new_base->lock);
-	spin_lock(&old_base->lock);
+	double_timer_lock(new_base, old_base, smp_processor_id() - cpu);
 
 	BUG_ON(old_base->running_timer);
 

@@ -1665,8 +1696,7 @@ static void __devinit migrate_timers(int
 		migrate_timer_list(new_base, old_base->tv5.vec + i);
 	}
 
-	spin_unlock(&old_base->lock);
-	spin_unlock(&new_base->lock);
+	double_timer_unlock(new_base, old_base, smp_processor_id() - cpu);
 	local_irq_enable();
 	put_cpu_var(tvec_bases);
 }

Sure. Will put a static inline function into include/linux/spinlock.h. "ind" is supposed to be the short form of "indicator".

From: Heiko Carstens <[email protected]>

Doing something like this on a two cpu system

# echo 0 > /sys/devices/system/cpu/cpu0/online 
# echo 1 > /sys/devices/system/cpu/cpu0/online 
# echo 0 > /sys/devices/system/cpu/cpu1/online 

will give me this:

=======================================================
[ INFO: possible circular locking dependency detected ] 2.6.21-rc2-g562aa1d4-dirty #7
-------------------------------------------------------
bash/1282 is trying to acquire lock:
(&cpu_base->lock_key){.+..}, at: [<000000000005f17e>] hrtimer_cpu_notify+0xc6/0x240

but task is already holding lock:
(&cpu_base->lock_key#2){.+..}, at: [<000000000005f174>] hrtimer_cpu_notify+0xbc/0x240

which lock already depends on the new lock.

This happens because we have the following code in kernel/hrtimer.c:

migrate_hrtimers(int cpu)
[...]
old_base = &per_cpu(hrtimer_bases, cpu);
new_base = &get_cpu_var(hrtimer_bases);
[...]
spin_lock(&new_base->lock);
spin_lock(&old_base->lock);

Which means the spinlocks are taken in an order which depends on which cpu gets shut down from which other cpu. Therefore lockdep complains that there might be an ABBA deadlock. Since migrate_hrtimers() gets only called on cpu hotplug it's safe to assume that it isn't executed concurrently on a different cpu and therefore the locking should be ok.

The same problem exists in kernel/timer.c: migrate_timers().

As pointed out by Christian Borntraeger one possible solution to avoid the locking order complaints would be to make sure that the locks are always taken in the same order. E.g. by taking the lock of the cpu with the lower number first.

To achieve this we introduce two new spinlock functions double_spin_lock and double_spin_unlock which lock or unlock two locks in a given order.

---

Next try. Hopefully the interface with "l1_first" and "l1_taken_first" is not confusing?

 include/linux/spinlock.h |   37 +++++++++++++++++++++++++++++++++++++
 kernel/hrtimer.c         |    9 ++++-----
 kernel/timer.c           |    8 ++++----
 3 files changed, 45 insertions(+), 9 deletions(-)

Index: linux-2.6/kernel/hrtimer.c
===================================================================
@@ -1355,17 +1355,16 @@ static void migrate_hrtimers(int cpu)
 	tick_cancel_sched_timer(cpu);
 
 	local_irq_disable();
-
-	spin_lock(&new_base->lock);
-	spin_lock(&old_base->lock);
+	double_spin_lock(&new_base->lock, &old_base->lock,
+			 smp_processor_id() < cpu);
 
 	for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
 		migrate_hrtimer_list(&old_base->clock_base[i],
 				     &new_base->clock_base[i]);
 	}
-	spin_unlock(&old_base->lock);
-	spin_unlock(&new_base->lock);
 
+	double_spin_unlock(&new_base->lock, &old_base->lock,
+			   smp_processor_id() < cpu);
 	local_irq_enable();
 	put_cpu_var(hrtimer_bases);
 }

Index: linux-2.6/kernel/timer.c
===================================================================
@@ -1651,8 +1651,8 @@ static void __devinit migrate_timers(int
 	new_base = get_cpu_var(tvec_bases);
 
 	local_irq_disable();
-	spin_lock(&new_base->lock);
-	spin_lock(&old_base->lock);
+	double_spin_lock(&new_base->lock, &old_base->lock,
+			 smp_processor_id() < cpu);
 
 	BUG_ON(old_base->running_timer);
 

@@ -1665,8 +1665,8 @@ static void __devinit migrate_timers(int
 		migrate_timer_list(new_base, old_base->tv5.vec + i);
 	}
 
-	spin_unlock(&old_base->lock);
-	spin_unlock(&new_base->lock);
+	double_spin_unlock(&new_base->lock, &old_base->lock,
+			   smp_processor_id() < cpu);
 	local_irq_enable();
 	put_cpu_var(tvec_bases);
 }

Index: linux-2.6/include/linux/spinlock.h
===================================================================
@@ -283,6 +283,43 @@ do {						\
 })
 
 /*
+ * Locks two spinlocks l1 and l2.
+ * l1_first indicates if spinlock l1 should be taken first.
+ */
+static inline void double_spin_lock(spinlock_t *l1, spinlock_t *l2,
+				    bool l1_first)
+	__acquires(l1)
+	__acquires(l2)
+{
+	if (l1_first) {
+		spin_lock(l1);
+		spin_lock(l2);
+	} else {
+		spin_lock(l2);
+		spin_lock(l1);
+	}
+}
+
+/*
+ * Unlocks two spinlocks l1 and l2.
+ * l1_taken_first indicates if spinlock l1 was taken first and therefore
+ * should be released after spinlock l2.
+ */
+static inline void double_spin_unlock(spinlock_t *l1, spinlock_t *l2,
+				      bool l1_taken_first)
+	__releases(l1)
+	__releases(l2)
+{
+	if (l1_taken_first) {
+		spin_unlock(l2);
+		spin_unlock(l1);
+	} else {
+		spin_unlock(l1);
+		spin_unlock(l2);
+	}
+}
+
+/*
  * Pull the atomic_t declaration:
  * (asm-mips/atomic.h needs above definitions)
  */