[PATCH RT 19/32] locking/rtmutex: Clean ->pi_blocked_on in the error case

Steven Rostedt
2020-01-17T17:42:27Z

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 0be4ea6e3ce693101be0fbd55a0cc7ce238ab2eb ]

The function rt_mutex_wait_proxy_lock() cleans ->pi_blocked_on in case
of failure (timeout, signal). The same cleanup is required in
__rt_mutex_start_proxy_lock().
In both the cases the tasks was interrupted by a signal or timeout while
acquiring the lock and after the interruption it longer blocks on the
lock.

Fixes: 1a1fb985f2e2b ("futex: Handle early deadlock return correctly")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/locking/rtmutex.c | 43 +++++++++++++++++++++++-----------------
1 file changed, 25 insertions(+), 18 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 2a9bf2443acc..63b3d6f306fa 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -2320,6 +2320,26 @@ void rt_mutex_proxy_unlock(struct rt_mutex *lock,
rt_mutex_set_owner(lock, NULL);
}

+static void fixup_rt_mutex_blocked(struct rt_mutex *lock)
+{
+ struct task_struct *tsk = current;
+ /*
+ * RT has a problem here when the wait got interrupted by a timeout
+ * or a signal. task->pi_blocked_on is still set. The task must
+ * acquire the hash bucket lock when returning from this function.
+ *
+ * If the hash bucket lock is contended then the
+ * BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on)) in
+ * task_blocks_on_rt_mutex() will trigger. This can be avoided by
+ * clearing task->pi_blocked_on which removes the task from the
+ * boosting chain of the rtmutex. That's correct because the task
+ * is not longer blocked on it.
+ */
+ raw_spin_lock(&tsk->pi_lock);
+ tsk->pi_blocked_on = NULL;
+ raw_spin_unlock(&tsk->pi_lock);
+}
+
/**
* __rt_mutex_start_proxy_lock() - Start lock acquisition for another task
* @lock: the rt_mutex to take
@@ -2392,6 +2412,9 @@ int __rt_mutex_start_proxy_lock(struct rt_mutex *lock,
ret = 0;
}

+ if (ret)
+ fixup_rt_mutex_blocked(lock);
+
debug_rt_mutex_print_deadlock(waiter);

return ret;
@@ -2472,7 +2495,6 @@ int rt_mutex_wait_proxy_lock(struct rt_mutex *lock,
struct hrtimer_sleeper *to,
struct rt_mutex_waiter *waiter)
{
- struct task_struct *tsk = current;
int ret;

raw_spin_lock_irq(&lock->wait_lock);
@@ -2484,23 +2506,8 @@ int rt_mutex_wait_proxy_lock(struct rt_mutex *lock,
* have to fix that up.
*/
fixup_rt_mutex_waiters(lock);
- /*
- * RT has a problem here when the wait got interrupted by a timeout
- * or a signal. task->pi_blocked_on is still set. The task must
- * acquire the hash bucket lock when returning from this function.
- *
- * If the hash bucket lock is contended then the
- * BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on)) in
- * task_blocks_on_rt_mutex() will trigger. This can be avoided by
- * clearing task->pi_blocked_on which removes the task from the
- * boosting chain of the rtmutex. That's correct because the task
- * is not longer blocked on it.
- */
- if (ret) {
- raw_spin_lock(&tsk->pi_lock);
- tsk->pi_blocked_on = NULL;
- raw_spin_unlock(&tsk->pi_lock);
- }
+ if (ret)
+ fixup_rt_mutex_blocked(lock);

raw_spin_unlock_irq(&lock->wait_lock);

--
2.24.1