Messages in this thread | | | Date | Mon, 29 Mar 2010 12:21:59 -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 Mon, Mar 29, 2010 at 08:02:28PM +0100, David Howells wrote: > Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > > > > spin_lock(&clp->cl_lock); > > > - if (rcu_dereference(nfsi->delegation) != NULL) { > > > + if (nfsi->delegation != NULL) { > > > > And this one. I thought that Trond said that clp->cl_lock protects > > this one, in which case this should work: > > > > 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.
> > > - if (rcu_dereference(nfsi->delegation) != NULL) { > > > + if (nfsi->delegation != NULL) { > > > > And this one, although the check for cp->cl_lock obviously won't work here. > > > > > spin_lock(&clp->cl_lock); > > > delegation = nfs_detach_delegation_locked(nfsi, NULL); > > > spin_unlock(&clp->cl_lock); > > On this one, why does nfsi->delegation need a memory barrier interpolating > afterwards? It has an implicit one in the form of the spin_lock() immediately > after, if the value of the pointer wasn't NULL. What two memory accesses is > the memory barrier ordering? > > Ditto on the next one.
I must defer to Trond on this one.
Thanx, Paul
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