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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-05-04 15:38:38 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-07-09 09:14:07 -0700
commitf27bc4873fa8b75cc1eba7b641eda7375dc72ccf (patch)
treee5652f17806cb0b93fe5d5b22f020acf61475854 /include/linux/rcupdate.h
parent4a81e8328d3791a4f99bf5b436d050f6dc5ffea3 (diff)
rcu: Document deadlock-avoidance information for rcu_read_unlock()
Reported-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Diffstat (limited to 'include/linux/rcupdate.h')
-rw-r--r--include/linux/rcupdate.h28
1 files changed, 28 insertions, 0 deletions
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 6a94cc8b1ca0..c56ad15204ec 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -858,6 +858,34 @@ static inline void rcu_read_lock(void)
/**
* 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.
*/
static inline void rcu_read_unlock(void)