migrate_timers: add comment, use spinlock_irq()

Add the comment to explain why the double lock in migrate_timers()
can't deadlock.

Change the code to use spinlock_irq() instead of local_irq_disable()
+ spin_lock().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 12 insertions, 10 deletions

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index b8e4dce80a74..03ea1378c43b 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -1620,9 +1620,11 @@ static void migrate_hrtimers(int cpu)
         new_base = &get_cpu_var(hrtimer_bases);
 
         tick_cancel_sched_timer(cpu);
-
-        local_irq_disable();
-        spin_lock(&new_base->lock);
+        /*
+         * The caller is globally serialized and nobody else
+         * takes two locks at once, deadlock is not possible.
+         */
+        spin_lock_irq(&new_base->lock);
         spin_lock_nested(&old_base->lock, SINGLE_DEPTH_NESTING);
 
         for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
@@ -1631,8 +1633,7 @@ static void migrate_hrtimers(int cpu)
         }
 
         spin_unlock(&old_base->lock);
-        spin_unlock(&new_base->lock);
-        local_irq_enable();
+        spin_unlock_irq(&new_base->lock);
         put_cpu_var(hrtimer_bases);
 }
 #endif /* CONFIG_HOTPLUG_CPU */
diff --git a/kernel/timer.c b/kernel/timer.c
index 03bc7f1f1593..e8019cc3418d 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1435,9 +1435,11 @@ static void __cpuinit migrate_timers(int cpu)
         BUG_ON(cpu_online(cpu));
         old_base = per_cpu(tvec_bases, cpu);
         new_base = get_cpu_var(tvec_bases);
-
-        local_irq_disable();
-        spin_lock(&new_base->lock);
+        /*
+         * The caller is globally serialized and nobody else
+         * takes two locks at once, deadlock is not possible.
+         */
+        spin_lock_irq(&new_base->lock);
         spin_lock_nested(&old_base->lock, SINGLE_DEPTH_NESTING);
 
         BUG_ON(old_base->running_timer);
@@ -1452,8 +1454,7 @@ static void __cpuinit migrate_timers(int cpu)
         }
 
         spin_unlock(&old_base->lock);
-        spin_unlock(&new_base->lock);
-        local_irq_enable();
+        spin_unlock_irq(&new_base->lock);
         put_cpu_var(tvec_bases);
 }
 #endif /* CONFIG_HOTPLUG_CPU */