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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2012-01-12 17:21:20 -0800
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2012-02-21 09:03:46 -0800
commit5e1ee6e1016763812018bf5c5e966992821dc47e (patch)
treef2acb5d76a39da392d1f43e5c78a5ffc1482ef71 /include/linux/rcupdate.h
parent50406b98b6372e7de21d903d2cf3914e9d64e094 (diff)
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rcu: Note that rcu_access_pointer() can be used for teardown
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@redhat.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: David Howells <dhowells@redhat.com>
Diffstat (limited to 'include/linux/rcupdate.h')
-rw-r--r--include/linux/rcupdate.h7
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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