From: Paul E. McKenney Date: Sun, 8 Aug 2010 04:59:54 +0000 (-0700) Subject: rcu: update obsolete rcu_read_lock() comment. X-Git-Tag: v2.6.37-rc1~215^2~14^2~9 X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=9079fd7c2e06a92cf27d05224a1f478581916c5b;p=karo-tx-linux.git rcu: update obsolete rcu_read_lock() comment. The comment says that blocking is illegal in rcu_read_lock()-style RCU read-side critical sections, which is no longer entirely true given preemptible RCU. This commit provides a fix. Suggested-by: David Miller Signed-off-by: Paul E. McKenney --- diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 24b896649384..d7af96ef6fcf 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -458,7 +458,20 @@ extern int rcu_my_thread_group_empty(void); * will be deferred until the outermost RCU read-side critical section * completes. * - * It is illegal to block while in an RCU read-side critical section. + * You can avoid reading and understanding the next paragraph by + * following this rule: don't put anything in an rcu_read_lock() RCU + * 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 (CONFIG_PREEMPT_RT) 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) {