Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2]

Paul E. McKenney
2010-04-05T17:57:57Z

On Thu, Apr 01, 2010 at 03:46:51PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
>
> > So you have objected to needless memory barriers. How do you feel
> > about possibly needless ACCESS_ONCE() calls?
>
> That would work here since it shouldn't emit any excess instructions.

And here is the corresponding patch. Seem reasonable?

Thanx, Paul

------------------------------------------------------------------------

rcupdate.h | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

commit 61ad1405cd442fe54f87ff97febf52610817903e
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Mon Apr 5 10:52:53 2010 -0700

rcu: add rcu_dereference_protect to avoid smp_read_barrier_depends overhead

This patch adds a variant of rcu_dereference() that handles situations
where the RCU-protected data structure cannot change, perhaps due to
our holding the update-side lock, or where the RCU-protected pointer is
only to be tested, not dereferenced.

Suggested-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 872a98e..123b834 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -209,9 +209,28 @@ static inline int rcu_read_lock_sched_held(void)
rcu_dereference_raw(p); \
})

+/**
+ * rcu_dereference_protected - fetch RCU pointer when protected by algorithm
+ *
+ * Return the value of the specified RCU-protected pointer, but omit
+ * the smp_read_barrier_depends() and keep the ACCESS_ONCE(). This
+ * is useful in cases where update-side locks prevent the value of the
+ * pointer from changing, and is also useful in cases where the value
+ * of this pointer is accessed, but the pointer is not dereferenced.
+ * An example of this latter occurs when testing an RCU-protected
+ * pointer against NULL.
+ */
+#define rcu_dereference_protected(p, c) \
+ ({ \
+ if (debug_lockdep_rcu_enabled() && !(c)) \
+ lockdep_rcu_dereference(__FILE__, __LINE__); \
+ ACCESS_ONCE(p); \
+ })
+
#else /* #ifdef CONFIG_PROVE_RCU */

#define rcu_dereference_check(p, c) rcu_dereference_raw(p)
+#define rcu_dereference_protected(p, c) ACCESS_ONCE(p)

#endif /* #else #ifdef CONFIG_PROVE_RCU */