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Date:	Fri,  9 Apr 2010 15:39:11 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/urgent 2/3] RCU: Better explain the condition parameter of rcu_dereference_check()

From: David Howells <dhowells@...hat.com>

Better explain the condition parameter of rcu_dereference_check() that
describes the conditions under which the dereference is permitted to
take place (and incorporate Yong Zhang's suggestion).  This condition
is only checked under lockdep proving.

Signed-off-by: David Howells <dhowells@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 include/linux/rcupdate.h |   28 +++++++++++++++++++++++-----
 1 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 8fe8660..9f1ddfe 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -195,12 +195,30 @@ static inline int rcu_read_lock_sched_held(void)
 
 /**
  * rcu_dereference_check - rcu_dereference with debug checking
+ * @p: The pointer to read, prior to dereferencing
+ * @c: The conditions under which the dereference will take place
  *
- * Do an rcu_dereference(), but check that the context is correct.
- * For example, rcu_dereference_check(gp, rcu_read_lock_held()) to
- * ensure that the rcu_dereference_check() executes within an RCU
- * read-side critical section.  It is also possible to check for
- * locks being held, for example, by using lockdep_is_held().
+ * Do an rcu_dereference(), but check that the conditions under which the
+ * dereference will take place are correct.  Typically the conditions indicate
+ * the various locking conditions that should be held at that point.  The check
+ * should return true if the conditions are satisfied.
+ *
+ * For example:
+ *
+ *	bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||
+ *					      lockdep_is_held(&foo->lock));
+ *
+ * could be used to indicate to lockdep that foo->bar may only be dereferenced
+ * if either the RCU read lock is held, or that the lock required to replace
+ * the bar struct at foo->bar is held.
+ *
+ * Note that the list of conditions may also include indications of when a lock
+ * need not be held, for example during initialisation or destruction of the
+ * target struct:
+ *
+ *	bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||
+ *					      lockdep_is_held(&foo->lock) ||
+ *					      atomic_read(&foo->usage) == 0);
  */
 #define rcu_dereference_check(p, c) \
 	({ \
-- 
1.7.0

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