Commit-ID: f27bc4873fa8b75cc1eba7b641eda7375dc72ccf Gitweb: http://git.kernel.org/tip/f27bc4873fa8b75cc1eba7b641eda7375dc72ccf Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Sun, 4 May 2014 15:38:38 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Wed, 9 Jul 2014 09:14:07 -0700 rcu: Document deadlock-avoidance information for rcu_read_unlock() Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Reviewed-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx> --- include/linux/rcupdate.h | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 6a94cc8..c56ad15 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) -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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