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From: Darren Hart on 9 Jul 2010 18:40
rtmutex proxy locking complicates the logic a bit and opens up
the possibility for a task to wake and attempt to take another
sleeping lock without knowing it has been enqueued on another
lock already. Add a BUG_ON to catch this scenario early.

Signed-off-by: Darren Hart <dvhltc(a)us.ibm.com>
Cc: Thomas Gleixner <tglx(a)linutronix.de>
Cc: Peter Zijlstra <peterz(a)infradead.org>
Cc: Ingo Molnar <mingo(a)elte.hu>
Cc: Eric Dumazet <eric.dumazet(a)gmail.com>
Cc: John Kacur <jkacur(a)redhat.com>
Cc: Steven Rostedt <rostedt(a)goodmis.org>
Cc: Mike Galbraith <efault(a)gmx.de>
---
kernel/rtmutex.c | 3 +++
1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index baac7d9..22f9d18 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -459,6 +459,9 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
top_waiter = rt_mutex_top_waiter(lock);
plist_add(&waiter->list_entry, &lock->wait_list);

+ /* Tasks can only block on one lock at a time. */
+ BUG_ON(task->pi_blocked_on != NULL);
+
task->pi_blocked_on = waiter;

raw_spin_unlock(&task->pi_lock);
--
1.7.0.4

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Prev: pidns: Remove races by stopping the caching of proc_mnt
Next: [PATCH 1/4] rtmutex: avoid null derefence in WARN_ON