[PATCH v5] CPU hotplug: active_writer not woken up in some cases - deadlock

David Hildenbrand
2014-12-12T09:11:57Z

Commit b2c4623dcd07 ("rcu: More on deadlock between CPU hotplug and expedited
grace periods") introduced another problem that can easily be reproduced by
starting/stopping cpus in a loop.

E.g.:
for i in `seq 5000`; do
echo 1 > /sys/devices/system/cpu/cpu1/online
echo 0 > /sys/devices/system/cpu/cpu1/online
done

Will result in:
INFO: task /cpu_start_stop:1 blocked for more than 120 seconds.
Call Trace:
([<00000000006a028e>] __schedule+0x406/0x91c)
[<0000000000130f60>] cpu_hotplug_begin+0xd0/0xd4
[<0000000000130ff6>] _cpu_up+0x3e/0x1c4
[<0000000000131232>] cpu_up+0xb6/0xd4
[<00000000004a5720>] device_online+0x80/0xc0
[<00000000004a57f0>] online_store+0x90/0xb0
...

And a deadlock.

Problem is that if the last ref in put_online_cpus() can't get the
cpu_hotplug.lock the puts_pending count is incremented, but a sleeping
active_writer might never be woken up, therefore never exiting the loop in
cpu_hotplug_begin().

This fix removes puts_pending and turns refcount into an atomic variable. We
also introduce a wait queue for the active_writer, to avoid possible races and
use-after-free. There is no need to take the lock in put_online_cpus() anymore.

Can't reproduce it with this fix.

Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
---
kernel/cpu.c | 56 +++++++++++++++++++++++---------------------------------
1 file changed, 23 insertions(+), 33 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5d22023..1972b16 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -58,22 +58,23 @@ static int cpu_hotplug_disabled;

static struct {
struct task_struct *active_writer;
- struct mutex lock; /* Synchronizes accesses to refcount, */
+ /* wait queue to wake up the active_writer */
+ wait_queue_head_t wq;
+ /* verifies that no writer will get active while readers are active */
+ struct mutex lock;
/*
* Also blocks the new readers during
* an ongoing cpu hotplug operation.
*/
- int refcount;
- /* And allows lockless put_online_cpus(). */
- atomic_t puts_pending;
+ atomic_t refcount;

#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
} cpu_hotplug = {
.active_writer = NULL,
+ .wq = __WAIT_QUEUE_HEAD_INITIALIZER(cpu_hotplug.wq),
.lock = __MUTEX_INITIALIZER(cpu_hotplug.lock),
- .refcount = 0,
#ifdef CONFIG_DEBUG_LOCK_ALLOC
.dep_map = {.name = "cpu_hotplug.lock" },
#endif
@@ -86,15 +87,6 @@ static struct {
#define cpuhp_lock_acquire() lock_map_acquire(&cpu_hotplug.dep_map)
#define cpuhp_lock_release() lock_map_release(&cpu_hotplug.dep_map)

-static void apply_puts_pending(int max)
-{
- int delta;
-
- if (atomic_read(&cpu_hotplug.puts_pending) >= max) {
- delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
- cpu_hotplug.refcount -= delta;
- }
-}

void get_online_cpus(void)
{
@@ -103,8 +95,7 @@ void get_online_cpus(void)
return;
cpuhp_lock_acquire_read();
mutex_lock(&cpu_hotplug.lock);
- apply_puts_pending(65536);
- cpu_hotplug.refcount++;
+ atomic_inc(&cpu_hotplug.refcount);
mutex_unlock(&cpu_hotplug.lock);
}
EXPORT_SYMBOL_GPL(get_online_cpus);
@@ -116,8 +107,7 @@ bool try_get_online_cpus(void)
if (!mutex_trylock(&cpu_hotplug.lock))
return false;
cpuhp_lock_acquire_tryread();
- apply_puts_pending(65536);
- cpu_hotplug.refcount++;
+ atomic_inc(&cpu_hotplug.refcount);
mutex_unlock(&cpu_hotplug.lock);
return true;
}
@@ -125,20 +115,18 @@ EXPORT_SYMBOL_GPL(try_get_online_cpus);

void put_online_cpus(void)
{
+ int refcount;
+
if (cpu_hotplug.active_writer == current)
return;
- if (!mutex_trylock(&cpu_hotplug.lock)) {
- atomic_inc(&cpu_hotplug.puts_pending);
- cpuhp_lock_release();
- return;
- }

- if (WARN_ON(!cpu_hotplug.refcount))
- cpu_hotplug.refcount++; /* try to fix things up */
+ refcount = atomic_dec_return(&cpu_hotplug.refcount);
+ if (WARN_ON(refcount < 0)) /* try to fix things up */
+ atomic_inc(&cpu_hotplug.refcount);
+
+ if (refcount <= 0 && waitqueue_active(&cpu_hotplug.wq))
+ wake_up(&cpu_hotplug.wq);

- if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
- wake_up_process(cpu_hotplug.active_writer);
- mutex_unlock(&cpu_hotplug.lock);
cpuhp_lock_release();

}
@@ -168,18 +156,20 @@ EXPORT_SYMBOL_GPL(put_online_cpus);
*/
void cpu_hotplug_begin(void)
{
- cpu_hotplug.active_writer = current;
+ DEFINE_WAIT(wait);

+ cpu_hotplug.active_writer = current;
cpuhp_lock_acquire();
+
for (;;) {
mutex_lock(&cpu_hotplug.lock);
- apply_puts_pending(1);
- if (likely(!cpu_hotplug.refcount))
- break;
- __set_current_state(TASK_UNINTERRUPTIBLE);
+ prepare_to_wait(&cpu_hotplug.wq, &wait, TASK_UNINTERRUPTIBLE);
+ if (likely(!atomic_read(&cpu_hotplug.refcount)))
+ break;
mutex_unlock(&cpu_hotplug.lock);
schedule();
}
+ finish_wait(&cpu_hotplug.wq, &wait);
}

void cpu_hotplug_done(void)
--
1.8.5.5