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Subject[PATCH tip/core/rcu 33/55] rcu: Improve rcu_assign_pointer() and RCU_INIT_POINTER() documentation
Date
The differences between rcu_assign_pointer() and RCU_INIT_POINTER() are
subtle, and it is easy to use the the cheaper RCU_INIT_POINTER() when
the more-expensive rcu_assign_pointer() should have been used instead.
The consequences of this mistake are quite severe.

This commit therefore carefully lays out the situations in which it it
permissible to use RCU_INIT_POINTER().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
include/linux/rcupdate.h | 47 +++++++++++++++++++++++++++++++++++++++------
1 files changed, 40 insertions(+), 7 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index b2e5fe8..9873040 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -754,11 +754,18 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
* any prior initialization. Returns the value assigned.
*
* Inserts memory barriers on architectures that require them
- * (pretty much all of them other than x86), and also prevents
- * the compiler from reordering the code that initializes the
- * structure after the pointer assignment. More importantly, this
- * call documents which pointers will be dereferenced by RCU read-side
- * code.
+ * (which is most of them), and also prevents the compiler from
+ * reordering the code that initializes the structure after the pointer
+ * assignment. More importantly, this call documents which pointers
+ * will be dereferenced by RCU read-side code.
+ *
+ * In some special cases, you may use RCU_INIT_POINTER() instead
+ * of rcu_assign_pointer(). RCU_INIT_POINTER() is a bit faster due
+ * to the fact that it does not constrain either the CPU or the compiler.
+ * That said, using RCU_INIT_POINTER() when you should have used
+ * rcu_assign_pointer() is a very bad thing that results in
+ * impossible-to-diagnose memory corruption. So please be careful.
+ * See the RCU_INIT_POINTER() comment header for details.
*/
#define rcu_assign_pointer(p, v) \
__rcu_assign_pointer((p), (v), __rcu)
@@ -766,8 +773,34 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
/**
* RCU_INIT_POINTER() - initialize an RCU protected pointer
*
- * Initialize an RCU-protected pointer in such a way to avoid RCU-lockdep
- * splats.
+ * Initialize an RCU-protected pointer in special cases where readers
+ * do not need ordering constraints on the CPU or the compiler. These
+ * special cases are:
+ *
+ * 1. This use of RCU_INIT_POINTER() is NULLing out the pointer -or-
+ * 2. The caller has taken whatever steps are required to prevent
+ * RCU readers from concurrently accessing this pointer -or-
+ * 3. The referenced data structure has already been exposed to
+ * readers either at compile time or via rcu_assign_pointer() -and-
+ * a. You have not made -any- reader-visible changes to
+ * this structure since then -or-
+ * b. It is OK for readers accessing this structure from its
+ * new location to see the old state of the structure. (For
+ * example, the changes were to statistical counters or to
+ * other state where exact synchronization is not required.)
+ *
+ * Failure to follow these rules governing use of RCU_INIT_POINTER() will
+ * result in impossible-to-diagnose memory corruption. As in the structures
+ * will look OK in crash dumps, but any concurrent RCU readers might
+ * see pre-initialized values of the referenced data structure. So
+ * please be very careful how you use RCU_INIT_POINTER()!!!
+ *
+ * If you are creating an RCU-protected linked structure that is accessed
+ * by a single external-to-structure RCU-protected pointer, then you may
+ * use RCU_INIT_POINTER() to initialize the internal RCU-protected
+ * pointers, but you must use rcu_assign_pointer() to initialize the
+ * external-to-structure pointer -after- you have completely initialized
+ * the reader-accessible portions of the linked structure.
*/
#define RCU_INIT_POINTER(p, v) \
p = (typeof(*v) __force __rcu *)(v)
--
1.7.3.2


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