A task that boosts an owner of a lock via spin_trylock_or_boost() is not a
real waiter of the lock in non PREEMPT_RT code. In non PREEMPT_RT, that task
is just spinning. But in PREEMPT_RT the call to cpu_chill() will touch the
lock. But there's nothing keeping the lock there.
As the lock is boosted via a trylock, it means something had to be done
before we got that lock with another lock held out of reverse order. That
means, if there's nothing using that lock, there's a possible code path that
can make that lock disappear. Here's a fictitious example:
CPU0 CPU1 CPU2
---- ---- ----
[task0] [task1] [task2]
lock(dev->A)
lock(B)
trylock(dev->A)
unlock(B)
goto again
lock(B)
trylock(dev->A)
unlock(B)
goto again
unlock(dev->A)
wake(task1)
remove_task1_links
lock(B)
free(dev)
unlock(B)
At this moment, although task1 is running and ready to go, task2 is still on
dev->A->wait_list, and that will cause a panic when task2 does a cpu_chill().
Things are fine as long as there's a waiter that is from a rtmutex_lock().
Wake all the top tasks till a task is found that is blocked on the rtmutex
itself.
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
kernel/locking/rtmutex.c | 65 +++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 59 insertions(+), 6 deletions(-)
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 843b67f38e20..f26eebe5de87 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -254,17 +254,25 @@ rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
}
#ifdef CONFIG_PREEMPT_RT_FULL
+static void rt_mutex_wake_waiter(struct rt_mutex_waiter *waiter);
/*
- * Returns true if the task should be woken up, false otherwise.
+ * Returns true if this is a trylock waiter.
*/
static inline bool rt_mutex_wake_trylock_waiter(struct rt_mutex_waiter *waiter)
{
- struct task_struct *task = waiter->task;
+ struct task_struct *task;
+ struct rt_mutex *lock;
unsigned long flags;
bool wakeup;
+ bool trylock_waiter = false;
+
+again:
+ task = waiter->task;
if (likely(waiter != &task->rt_waiter))
- return true;
+ return trylock_waiter;
+
+ trylock_waiter = true;
/*
* A task boosted current because it is within a trylock spin.
@@ -276,12 +284,57 @@ static inline bool rt_mutex_wake_trylock_waiter(struct rt_mutex_waiter *waiter)
*/
raw_spin_lock_irqsave(&task->pi_lock, flags);
rt_mutex_dequeue(waiter->lock, waiter);
+ lock = waiter->lock;
waiter->lock = NULL;
wakeup = waiter->wake_up;
+ get_task_struct(task);
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
- return wakeup;
+ if (wakeup)
+ rt_mutex_wake_waiter(waiter);
+
+ put_task_struct(task);
+
+ /*
+ * All tasks that are trylock waiters need to be woken up,
+ * otherwise there's a chance that the lock may go away from
+ * under them. Here's the scenario:
+ *
+ * CPU0 CPU1 CPU2
+ * ---- ---- ----
+ * [task0] [task1] [task2]
+ *
+ * lock(dev->A)
+ * lock(B)
+ * trylock(dev->A)
+ * unlock(B)
+ * goto again
+ * lock(B)
+ * trylock(dev->A)
+ * unlock(B)
+ * goto again
+ * unlock(dev->A)
+ * wake(task1)
+ * remove_task1_links
+ * lock(B)
+ * free(dev)
+ * unlock(B)
+ *
+ * At this moment, although task1 is running and ready
+ * to go, task2 is still on dev->wait_list, and that will
+ * cause a panic when task2 does a cpu_chill().
+ *
+ * Things are fine as long as there's a waiter that is
+ * from a rtmutex_lock(). Keep waking tasks until we find
+ * a rtmutex_lock() waiter.
+ */
+
+ if (!rt_mutex_has_waiters(lock))
+ return true;
+
+ waiter = rt_mutex_top_waiter(lock);
+ goto again;
}
static void __rt_mutex_adjust_prio(struct task_struct *task);
@@ -496,7 +549,7 @@ static inline struct task_struct *trylock_boost_owner(struct rt_mutex *lock)
}
static inline bool rt_mutex_wake_trylock_waiter(struct rt_mutex_waiter *waiter)
{
- return true;
+ return false;
}
static inline bool check_static_waiter(struct task_struct *task,
struct rt_mutex *lock, bool ok)
@@ -1654,7 +1707,7 @@ static void wakeup_next_waiter(struct rt_mutex *lock)
raw_spin_unlock_irqrestore(¤t->pi_lock, flags);
- if (!rt_mutex_wake_trylock_waiter(waiter))
+ if (rt_mutex_wake_trylock_waiter(waiter))
return;
/*
--
2.5.1
--
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