rcu: More info about potential deadlocks with rcu_read_unlock()
The comment above rcu_read_unlock() explains the potential deadlock if the caller holds one of the locks taken by rt_mutex_unlock() paths, but it is not clear from this documentation that any lock which can be taken from interrupt can lead to deadlock as well and we need to take rt_mutex_lock() into account too. The problem is that rt_mutex_lock() takes wait_lock without disabling irqs, and thus an interrupt taking some LOCK can obviously race with rcu_read_unlock_special() called with the same LOCK held. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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@ -887,7 +887,9 @@ static inline void rcu_read_lock(void)
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* Unfortunately, this function acquires the scheduler's runqueue and
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* priority-inheritance spinlocks. This means that deadlock could result
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* if the caller of rcu_read_unlock() already holds one of these locks or
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* any lock that is ever acquired while holding them.
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* any lock that is ever acquired while holding them; or any lock which
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* can be taken from interrupt context because rcu_boost()->rt_mutex_lock()
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* does not disable irqs while taking ->wait_lock.
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*
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* That said, RCU readers are never priority boosted unless they were
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* preempted. Therefore, one way to avoid deadlock is to make sure
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