From f27bc4873fa8b75cc1eba7b641eda7375dc72ccf Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sun, 4 May 2014 15:38:38 -0700 Subject: rcu: Document deadlock-avoidance information for rcu_read_unlock() Reported-by: Oleg Nesterov Signed-off-by: Paul E. McKenney Reviewed-by: Lai Jiangshan --- include/linux/rcupdate.h | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) (limited to 'include/linux/rcupdate.h') diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 6a94cc8b1ca..c56ad15204e 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) -- cgit v1.2.3-70-g09d2 From ab74fdfd4e11ec040f21cf87edc14fc9f62cc934 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sun, 4 May 2014 15:41:21 -0700 Subject: rcu: Handle obsolete references to TINY_PREEMPT_RCU Signed-off-by: Paul E. McKenney Reviewed-by: Lai Jiangshan --- include/linux/rcupdate.h | 17 ++++++++--------- init/Kconfig | 2 +- 2 files changed, 9 insertions(+), 10 deletions(-) (limited to 'include/linux/rcupdate.h') diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index c56ad15204e..d231aa17b1d 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -826,15 +826,14 @@ static inline void rcu_preempt_sleep_check(void) * read-side critical section that would block in a !PREEMPT kernel. * But if you want the full story, read on! * - * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU), it - * is illegal to block while in an RCU read-side critical section. In - * preemptible RCU implementations (TREE_PREEMPT_RCU and TINY_PREEMPT_RCU) - * in CONFIG_PREEMPT kernel builds, RCU read-side critical sections may - * be preempted, but explicit blocking is illegal. Finally, in preemptible - * RCU implementations in real-time (with -rt patchset) kernel builds, - * RCU read-side critical sections may be preempted and they may also - * block, but only when acquiring spinlocks that are subject to priority - * inheritance. + * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU), + * it is illegal to block while in an RCU read-side critical section. + * In preemptible RCU implementations (TREE_PREEMPT_RCU) in CONFIG_PREEMPT + * kernel builds, RCU read-side critical sections may be preempted, + * but explicit blocking is illegal. Finally, in preemptible RCU + * implementations in real-time (with -rt patchset) kernel builds, RCU + * read-side critical sections may be preempted and they may also block, but + * only when acquiring spinlocks that are subject to priority inheritance. */ static inline void rcu_read_lock(void) { diff --git a/init/Kconfig b/init/Kconfig index 9d76b99af1b..977b37806e9 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -505,7 +505,7 @@ config PREEMPT_RCU def_bool TREE_PREEMPT_RCU help This option enables preemptible-RCU code that is common between - the TREE_PREEMPT_RCU and TINY_PREEMPT_RCU implementations. + TREE_PREEMPT_RCU and, in the old days, TINY_PREEMPT_RCU. config RCU_STALL_COMMON def_bool ( TREE_RCU || TREE_PREEMPT_RCU || RCU_TRACE ) -- cgit v1.2.3-70-g09d2