Commit f27bc487 authored by Paul E. McKenney's avatar Paul E. McKenney

rcu: Document deadlock-avoidance information for rcu_read_unlock()

Reported-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarLai Jiangshan <laijs@cn.fujitsu.com>
parent 4a81e832
...@@ -858,6 +858,34 @@ static inline void rcu_read_lock(void) ...@@ -858,6 +858,34 @@ static inline void rcu_read_lock(void)
/** /**
* rcu_read_unlock() - marks the end of an RCU read-side critical section. * rcu_read_unlock() - marks the end of an RCU read-side critical section.
* *
* In most situations, rcu_read_unlock() is immune from deadlock.
* However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
* is responsible for deboosting, which it does via rt_mutex_unlock().
* 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.
*
* That said, RCU readers are never priority boosted unless they were
* preempted. Therefore, one way to avoid deadlock is to make sure
* that preemption never happens within any RCU read-side critical
* section whose outermost rcu_read_unlock() is called with one of
* rt_mutex_unlock()'s locks held. Such preemption can be avoided in
* a number of ways, for example, by invoking preempt_disable() before
* critical section's outermost rcu_read_lock().
*
* Given that the set of locks acquired by rt_mutex_unlock() might change
* at any time, a somewhat more future-proofed approach is to make sure
* that that preemption never happens within any RCU read-side critical
* section whose outermost rcu_read_unlock() is called with irqs disabled.
* This approach relies on the fact that rt_mutex_unlock() currently only
* acquires irq-disabled locks.
*
* The second of these two approaches is best in most situations,
* however, the first approach can also be useful, at least to those
* developers willing to keep abreast of the set of locks acquired by
* rt_mutex_unlock().
*
* See rcu_read_lock() for more information. * See rcu_read_lock() for more information.
*/ */
static inline void rcu_read_unlock(void) static inline void rcu_read_unlock(void)
......
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