[PATCH tip/core/rcu 26/47] rcu: Note that rcu_access_pointer() can be used for teardown

From: Paul E. McKenney
Date: Fri Feb 03 2012 - 20:56:50 EST

From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

There is no convenient expression for rcu_deference_protected()
when it is used in tearing down multilinked structures following
a grace period. For example, suppose that an element containing an
RCU-protected pointer to a second element is removed from an enclosing
RCU-protected data structure, then the write-side lock is released,
and finally synchronize_rcu() is invoked to wait for a grace period.
Then it is necessary to traverse the pointer in order to free up the
second element. But we are not in an RCU read-side critical section
and we are holding no locks, so the usual rcu_dereference_check() and
rcu_dereference_protected() primitives are not appropriate. Neither
is rcu_dereference_raw(), as it is intended for use in data structures
where the user defines the locking design (for example, list_head).

So this responsibility is added to rcu_access_pointer()'s list, and
this commit updates rcu_assign_pointer()'s header comment accordingly.

Suggested-by: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Acked-by: David Howells <dhowells@xxxxxxxxxx>
include/linux/rcupdate.h | 7 +++++++
1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 6df0ae1..f409529 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -484,6 +484,13 @@ static inline void rcu_preempt_sleep_check(void)
* NULL. Although rcu_access_pointer() may also be used in cases where
* update-side locks prevent the value of the pointer from changing, you
* should instead use rcu_dereference_protected() for this use case.
+ *
+ * It is also permissible to use rcu_access_pointer() when read-side
+ * access to the pointer was removed at least one grace period ago, as
+ * is the case in the context of the RCU callback that is freeing up
+ * the data, or after a synchronize_rcu() returns. This can be useful
+ * when tearing down multi-linked structures after a grace period
+ * has elapsed.
#define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)


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