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authorDavid Howells <dhowells@redhat.com>2010-04-09 15:39:11 -0700
committerIngo Molnar <mingo@elte.hu>2010-04-14 12:20:04 +0200
commitc08c68dd76bd6b776bc0eb45a5e8f354ed772cdf (patch)
tree5324c73474b1e73631392de814e8ecebd5956d5b /include
parentb62730baea32f86fe91a7930e4b7ee8d82778b79 (diff)
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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>
Diffstat (limited to 'include')
-rw-r--r--include/linux/rcupdate.h28
1 files changed, 23 insertions, 5 deletions
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 8fe86609441..9f1ddfef84b 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) \
({ \