Messages in this thread Patch in this message | | | Date | Mon, 5 Apr 2010 10:57:54 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2] |
| |
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 */
| |