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Date: Wed, 10 Dec 2014 14:22:01 +0100
From: David Hildenbrand <dahi@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: heiko.carstens@...ibm.com, dahi@...ux.vnet.ibm.com,
borntraeger@...ibm.com, rafael.j.wysocki@...el.com,
paulmck@...ux.vnet.ibm.com, peterz@...radead.org, oleg@...hat.com,
bp@...e.de, jkosina@...e.cz
Subject: [PATCH v4] CPU hotplug: active_writer not woken up in some cases - deadlock
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.
Also rearrange the lockdep anotations so we won't get false positives.
Can't reproduce it with this fix.
Signed-off-by: David Hildenbrand <dahi@...ux.vnet.ibm.com>
---
kernel/cpu.c | 60 ++++++++++++++++++++++++++----------------------------------
1 file changed, 26 insertions(+), 34 deletions(-)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5d22023..3f1d5ba 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)
{
@@ -103,9 +95,9 @@ 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);
+ cpuhp_lock_release();
}
EXPORT_SYMBOL_GPL(get_online_cpus);
@@ -116,9 +108,9 @@ 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);
+ cpuhp_lock_release();
return true;
}
EXPORT_SYMBOL_GPL(try_get_online_cpus);
@@ -127,20 +119,16 @@ void put_online_cpus(void)
{
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 */
- if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
- wake_up_process(cpu_hotplug.active_writer);
- mutex_unlock(&cpu_hotplug.lock);
- cpuhp_lock_release();
+ if (atomic_dec_and_test(&cpu_hotplug.refcount) &&
+ waitqueue_active(&cpu_hotplug.wq))
+ wake_up(&cpu_hotplug.wq);
+ /* missing get - try to fix things up */
+ if (WARN_ON(atomic_read(&cpu_hotplug.refcount) < 0))
+ atomic_set(&cpu_hotplug.refcount, 0);
+ if (waitqueue_active(&cpu_hotplug.wq))
+ wake_up(&cpu_hotplug.wq);
}
EXPORT_SYMBOL_GPL(put_online_cpus);
@@ -168,18 +156,22 @@ EXPORT_SYMBOL_GPL(put_online_cpus);
*/
void cpu_hotplug_begin(void)
{
+ DEFINE_WAIT(wait);
+
cpu_hotplug.active_writer = current;
- cpuhp_lock_acquire();
for (;;) {
+ cpuhp_lock_acquire();
mutex_lock(&cpu_hotplug.lock);
- apply_puts_pending(1);
- if (likely(!cpu_hotplug.refcount))
+ prepare_to_wait(&cpu_hotplug.wq, &wait, TASK_UNINTERRUPTIBLE);
+ if (likely(!atomic_read(&cpu_hotplug.refcount)))
break;
- __set_current_state(TASK_UNINTERRUPTIBLE);
mutex_unlock(&cpu_hotplug.lock);
+ cpuhp_lock_release();
schedule();
}
+
+ finish_wait(&cpu_hotplug.wq, &wait);
}
void cpu_hotplug_done(void)
--
1.8.5.5
--
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