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rcu: update obsolete rcu_read_lock() comment.

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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 <davem@davemloft.net>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2010-08-07 21:59:54 -07:00
parent a57eb940d1
commit 9079fd7c2e
1 changed files with 14 additions and 1 deletions
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15
include/linux/rcupdate.h
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@ -458,7 +458,20 @@ extern int rcu_my_thread_group_empty(void);
* will be deferred until the outermost RCU read-side critical section * will be deferred until the outermost RCU read-side critical section
* completes. * 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) static inline void rcu_read_lock(void)
{ {