| From | "Paul E. McKenney" <> | Subject | [PATCH RFC tip/core/rcu 26/41] rcu: Note that rcu_access_pointer() can be used for teardown | Date | Wed, 1 Feb 2012 11:41:44 -0800 |
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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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> --- 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) -- 1.7.8
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