Gitweb:     http://git.kernel.org/linus/c08c68dd76bd6b776bc0eb45a5e8f354ed772cdf
Commit:     c08c68dd76bd6b776bc0eb45a5e8f354ed772cdf
Parent:     b62730baea32f86fe91a7930e4b7ee8d82778b79
Author:     David Howells <dhowells@redhat.com>
AuthorDate: Fri Apr 9 15:39:11 2010 -0700
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Wed Apr 14 12:20:04 2010 +0200

    rcu: Better explain the condition parameter of rcu_dereference_check()
    
    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@redhat.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: laijs@cn.fujitsu.com
    Cc: dipankar@in.ibm.com
    Cc: mathieu.desnoyers@polymtl.ca
    Cc: josh@joshtriplett.org
    Cc: dvhltc@us.ibm.com
    Cc: niv@us.ibm.com
    Cc: peterz@infradead.org
    Cc: rostedt@goodmis.org
    Cc: Valdis.Kletnieks@vt.edu
    Cc: eric.dumazet@gmail.com
    LKML-Reference: <1270852752-25278-2-git-send-email-paulmck@linux.vnet.ibm.com>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 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) \
 	({ \
--
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