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Groups > linux.kernel > #1533575

[patch 4/4] rtmutex: Explain locking rules for rt_mutex_proxy_unlock()/init_proxy_locked()

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While debugging the unlock vs. dequeue race which resulted in state
corruption of futexes the lockless nature of rt_mutex_proxy_unlock()
caused some confusion.

Add commentry to explain why it is safe to do this lockless. Add matching
comments to rt_mutex_init_proxy_locked() for completeness sake.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/locking/rtmutex.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1617,11 +1617,15 @@ EXPORT_SYMBOL_GPL(__rt_mutex_init);
  * rt_mutex_init_proxy_locked - initialize and lock a rt_mutex on behalf of a
  *				proxy owner
  *
- * @lock: 	the rt_mutex to be locked
+ * @lock:	the rt_mutex to be locked
  * @proxy_owner:the task to set as owner
  *
  * No locking. Caller has to do serializing itself
- * Special API call for PI-futex support
+ *
+ * Special API call for PI-futex support. This initializes the rtmutex and
+ * assigns it to @proxy_owner. Concurrent operations on the rtmutex are not
+ * possible at this point because the pi_state which contains the rtmutex
+ * is not yet visible to other tasks.
  */
 void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
 				struct task_struct *proxy_owner)
@@ -1635,10 +1639,14 @@ void rt_mutex_init_proxy_locked(struct r
 /**
  * rt_mutex_proxy_unlock - release a lock on behalf of owner
  *
- * @lock: 	the rt_mutex to be locked
+ * @lock:	the rt_mutex to be locked
  *
  * No locking. Caller has to do serializing itself
- * Special API call for PI-futex support
+ *
+ * Special API call for PI-futex support. This merily cleans up the rtmutex
+ * (debugging) state. Concurrent operations on this rt_mutex are not
+ * possible because it belongs to the pi_state which is about to be freed
+ * and it is not longer visible to other tasks.
  */
 void rt_mutex_proxy_unlock(struct rt_mutex *lock,
 			   struct task_struct *proxy_owner)

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[patch 0/4] rtmutex: Plug unlock vs. requeue race Thomas Gleixner <tglx@linutronix.de> - 2016-11-30 22:10 +0100
  [patch 4/4] rtmutex: Explain locking rules for  rt_mutex_proxy_unlock()/init_proxy_locked() Thomas Gleixner <tglx@linutronix.de> - 2016-11-30 22:10 +0100
    [tip:locking/core] locking/rtmutex: Explain locking rules for  rt_mutex_proxy_unlock()/init_proxy_locked() tip-bot for Thomas Gleixner <tipbot@zytor.com> - 2016-12-02 12:20 +0100
  [patch 2/4] rtmutex: Use READ_ONCE() in rt_mutex_owner() Thomas Gleixner <tglx@linutronix.de> - 2016-11-30 22:10 +0100
    [tip:locking/core] locking/rtmutex: Use READ_ONCE() in  rt_mutex_owner() tip-bot for Thomas Gleixner <tipbot@zytor.com> - 2016-12-02 11:50 +0100
  Re: [patch 0/4] rtmutex: Plug unlock vs. requeue race Peter Zijlstra <peterz@infradead.org> - 2016-12-01 19:40 +0100

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