nfs_localio: use cmpxchg() to install new nfs_file_localio

Rather than using nfs_uuid.lock to protect installing
a new ro_file or rw_file, change to use cmpxchg().
Removing the file already uses xchg() so this improves symmetry
and also makes the code a little simpler.

Also remove the optimisation of not taking the lock, and not removing
the nfs_file_localio from the linked list, when both ->ro_file and
->rw_file are already NULL.  Given that ->nfs_uuid was not NULL, it is
extremely unlikely that neither ->ro_file or ->rw_file is NULL so
this optimisation can be of little value and it complicates
understanding of the code - why can the list_del_init() be skipped?

Finally, move the assignment of NULL to ->nfs_uuid until after
the last action on the nfs_file_localio (the list_del_init).  As soon as
this is NULL a racing nfs_close_local_fh() can bypass all the locking
and go on to free the nfs_file_localio, so we must be certain to be
finished with it first.

Fixes: 86e00412254a ("nfs: cache all open LOCALIO nfsd_file(s) in client")
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

diff --git a/fs/nfs/localio.c b/fs/nfs/localio.c
index a4bacd9a50520a02cc15cefd3bf22c40a18edcf3..7a33da477da3e84d0f40dfe8654e9d0eab8128eb 100644 (file)
--- a/fs/nfs/localio.c
+++ b/fs/nfs/localio.c
@@ -282,14 +282,9 @@ nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred,
                        return NULL;
                rcu_read_lock();
                /* try to swap in the pointer */
-               spin_lock(&clp->cl_uuid.lock);
-               nf = rcu_dereference_protected(*pnf, 1);
-               if (!nf) {
-                       nf = new;
-                       new = NULL;
-                       rcu_assign_pointer(*pnf, nf);
-               }
-               spin_unlock(&clp->cl_uuid.lock);
+               nf = unrcu_pointer(cmpxchg(pnf, NULL, RCU_INITIALIZER(new)));
+               if (!nf)
+                       swap(nf, new);
        }
        nf = nfs_local_file_get(nf);
        rcu_read_unlock();

diff --git a/fs/nfs_common/nfslocalio.c b/fs/nfs_common/nfslocalio.c
index 6a0bdea6d6449f84a512f277b246b8ba76755188..bdf251332b6b86e283a3b0edda0b27896461d2ef 100644 (file)
--- a/fs/nfs_common/nfslocalio.c
+++ b/fs/nfs_common/nfslocalio.c
@@ -273,8 +273,8 @@ EXPORT_SYMBOL_GPL(nfs_open_local_fh);
 
 void nfs_close_local_fh(struct nfs_file_localio *nfl)
 {
-       struct nfsd_file *ro_nf = NULL;
-       struct nfsd_file *rw_nf = NULL;
+       struct nfsd_file *ro_nf;
+       struct nfsd_file *rw_nf;
        nfs_uuid_t *nfs_uuid;
 
        rcu_read_lock();
@@ -285,28 +285,23 @@ void nfs_close_local_fh(struct nfs_file_localio *nfl)
                return;
        }
 
-       ro_nf = rcu_access_pointer(nfl->ro_file);
-       rw_nf = rcu_access_pointer(nfl->rw_file);
-       if (ro_nf || rw_nf) {
-               spin_lock(&nfs_uuid->lock);
-               if (ro_nf)
-                       ro_nf = rcu_dereference_protected(xchg(&nfl->ro_file, NULL), 1);
-               if (rw_nf)
-                       rw_nf = rcu_dereference_protected(xchg(&nfl->rw_file, NULL), 1);
-
-               /* Remove nfl from nfs_uuid->files list */
-               RCU_INIT_POINTER(nfl->nfs_uuid, NULL);
-               list_del_init(&nfl->list);
-               spin_unlock(&nfs_uuid->lock);
-               rcu_read_unlock();
+       ro_nf = unrcu_pointer(xchg(&nfl->ro_file, NULL));
+       rw_nf = unrcu_pointer(xchg(&nfl->rw_file, NULL));
 
-               if (ro_nf)
-                       nfs_to_nfsd_file_put_local(ro_nf);
-               if (rw_nf)
-                       nfs_to_nfsd_file_put_local(rw_nf);
-               return;
-       }
+       spin_lock(&nfs_uuid->lock);
+       /* Remove nfl from nfs_uuid->files list */
+       list_del_init(&nfl->list);
+       spin_unlock(&nfs_uuid->lock);
        rcu_read_unlock();
+       /* Now we can allow racing nfs_close_local_fh() to
+        * skip the locking.
+        */
+       RCU_INIT_POINTER(nfl->nfs_uuid, NULL);
+
+       if (ro_nf)
+               nfs_to_nfsd_file_put_local(ro_nf);
+       if (rw_nf)
+               nfs_to_nfsd_file_put_local(rw_nf);
 }
 EXPORT_SYMBOL_GPL(nfs_close_local_fh);