Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2] | Date | Mon, 29 Mar 2010 21:15:06 +0100 |
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Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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.
David
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