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locking/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>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: David Daney <ddaney@caviumnetworks.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/20161130210030.591941927@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Thomas Gleixner 2016-11-30 21:04:45 +00:00 committed by Ingo Molnar
parent b5016e8203
commit 84d82ec5b9
1 changed files with 12 additions and 4 deletions
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16
kernel/locking/rtmutex.c
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@ -1619,11 +1619,15 @@ 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)
@ -1637,10 +1641,14 @@ void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
/**
* 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 merrily 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)