Gitweb:
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=e81ce1f7ecdaed2844c75313b09af791d44e6373
Commit: e81ce1f7ecdaed2844c75313b09af791d44e6373
Parent: 6bb74df481223731af6c7e0ff3adb31f6442cfcd
Author: Heiko Carstens <[EMAIL PROTECTED]>
AuthorDate: Mon Mar 5 00:30:51 2007 -0800
Committer: Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Mon Mar 5 07:57:53 2007 -0800
[PATCH] timer/hrtimer: take per cpu locks in sane order
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
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.
Cc: Ingo Molnar <[EMAIL PROTECTED]>
Cc: Thomas Gleixner <[EMAIL PROTECTED]>
Cc: Roman Zippel <[EMAIL PROTECTED]>
Cc: John Stultz <[EMAIL PROTECTED]>
Cc: Christian Borntraeger <[EMAIL PROTECTED]>
Cc: Martin Schwidefsky <[EMAIL PROTECTED]>
Signed-off-by: Heiko Carstens <[EMAIL PROTECTED]>
Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
---
include/linux/spinlock.h | 37 +++++++++++++++++++++++++++++++++++++
kernel/hrtimer.c | 9 ++++-----
kernel/timer.c | 8 ++++----
3 files changed, 45 insertions(+), 9 deletions(-)
diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 61fef37..a946176 100644
--- a/include/linux/spinlock.h
+++ b/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)
*/
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 476cb0c..de93a81 100644
--- a/kernel/hrtimer.c
+++ b/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);
}
diff --git a/kernel/timer.c b/kernel/timer.c
index 6663a87..8ad3842 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1651,8 +1651,8 @@ static void __devinit migrate_timers(int cpu)
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 cpu)
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);
}
-
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