Messages in this thread | | | Date | Thu, 1 Apr 2010 07:39:44 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2] |
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On Thu, Apr 01, 2010 at 12:45:14PM +0100, David Howells wrote: > Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > > > > I think it is incorrectly used. Given that the rcu_dereference() in: > > > > > > if (rcu_dereference(nfsi->delegation) != NULL) { > > > spin_lock(&clp->cl_lock); > > > delegation = nfs_detach_delegation_locked(nfsi, NULL); > > > spin_unlock(&clp->cl_lock); > > > if (delegation != NULL) > > > nfs_do_return_delegation(inode, delegation, 0); > > > } > > > > And nfs_detach_delegation_locked() rechecks nfsi->delegation() under > > the lock, so this is a legitimate use. > > > > The pointer is not held constant, but any changes will be accounted > > for and handled correctly. So I would argue that the pointer value is > > in fact protected by the recheck-under-lock algorithm used here. > > A legitimate use of what?
A legitimate use of loading an RCU-protected pointer without smp_read_barrier_depends(). However, I could imagine some situations where the ACCESS_ONCE() semantics were required -- though in this particular situation, I am having a hard time seeing how the compiler could mess us up. That said, my time on the C++ standards committee has given me new respect for the perversity of compiler writers.
So you have objected to needless memory barriers. How do you feel about possibly needless ACCESS_ONCE() calls?
Thanx, Paul
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