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

Paul E. McKenney
2010-03-29T23:26:38Z

On Mon, Mar 29, 2010 at 11:59:03PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
>
> > Only on Alpha. Otherwise only a volatile access.
>
> Whilst that is true, it's the principle of the thing. The extra barrier
> shouldn't be emitted on Alpha. If Alpha's no longer important, then can we
> scrap smp_read_barrier_depends()?
>
> My point is that some of these rcu_dereference*()'s are unnecessary. If
> there're required for correctness tracking purposes, fine; but can we have a
> macro that is just a dummy for the purpose of stripping the pointer Sparse
> annotation? One that doesn't invoke rcu_dereference_raw() and interpolate a
> barrier, pretend or otherwise, when there's no second reference to order
> against.

Interesting point. Perhaps an rcu_dereference_update(p, c) for the
cases where the data structure cannot change. Also, such a name makes
it more clear that this is an update-side access, and it further documents
the update-side lock.

Something like the following, then? Untested, probably does not even
compile.

Thanx, Paul

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

rcu: Add update-side variant of rcu_dereference()

Upcoming consistency-checking features are requiring that even update-side
accesses to RCU-protected pointers use some variant of rcu_dereference().
Even though rcu_dereference() is quite lightweight, it does constrain the
compiler, thus producing code that is worse than required. This patch
therefore adds rcu_dereference_update(), which allows lockdep-style
checks for holding the correct update-side lock, but which does not
constrain the compiler.

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

rcupdate.h | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 872a98e..0a6047f 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -209,9 +209,26 @@ static inline int rcu_read_lock_sched_held(void)
rcu_dereference_raw(p); \
})

+/**
+ * rcu_dereference_update - rcu_dereference on structure that cannot change
+ *
+ * Do rcu_dereference() checking, but just pick up the pointer without
+ * the normal RCU read-side precautions. These precautions are only
+ * needed if the data structure can change. The caller is responsible
+ * for doing whatever is necessary (such as holding locks) to prevent
+ * such changes from occurring.
+ */
+#define rcu_dereference_update(p, c) \
+ ({ \
+ if (debug_lockdep_rcu_enabled() && !(c)) \
+ lockdep_rcu_dereference(__FILE__, __LINE__); \
+ (p); \
+ })
+
#else /* #ifdef CONFIG_PROVE_RCU */

#define rcu_dereference_check(p, c) rcu_dereference_raw(p)
+#define rcu_dereference_update(p, c) (p)

#endif /* #else #ifdef CONFIG_PROVE_RCU */