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    SubjectRe: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2]
    Paul E. McKenney <> wrote:

    > > > if (rcu_dereference_check(nfsi->delegation,
    > > > lockdep_is_held(&clp->cl_lock)) != NULL) {
    > >
    > > If clp->cl_lock protects this pointer, why the need for
    > > rcu_dereference_check() at all? The check is redundant since the line
    > > above gets the very lock we're checking for.
    > Because Arnd Bergmann is working on a set of patches that makes sparse
    > complain if you access an RCU-protected pointer directly, without using
    > some flavor of rcu_dereference().
    > So your approach would work for the moment, but would need another
    > change, probably in the 2.6.35 timeframe.

    My objection to using rcu_dereference_check() here is that it's a dynamic
    check: the compiler emits code to do it, since the lock/unlock status of what
    the pointer points to cannot be determined easily at compiler time - and then
    the barrier is interpolated anyway unnecessarily.


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