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rcu: More info about potential deadlocks with rcu_read_unlock()

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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>
This commit is contained in:
Oleg Nesterov 2014-09-28 23:44:21 +02:00 committed by Paul E. McKenney
parent b6331ae8af
commit ce36f2f3eb
1 changed files with 3 additions and 1 deletions
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4
include/linux/rcupdate.h
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@ -887,7 +887,9 @@ static inline void rcu_read_lock(void)
* Unfortunately, this function acquires the scheduler's runqueue and
* priority-inheritance spinlocks. This means that deadlock could result
* if the caller of rcu_read_unlock() already holds one of these locks or
* any lock that is ever acquired while holding them.
* any lock that is ever acquired while holding them; or any lock which
* can be taken from interrupt context because rcu_boost()->rt_mutex_lock()
* does not disable irqs while taking ->wait_lock.
*
* That said, RCU readers are never priority boosted unless they were
* preempted. Therefore, one way to avoid deadlock is to make sure