[PATCH 1/6] nfs_localio: use cmpxchg() to install new nfs_file_localio

NeilBrown
2025-05-09T00:49:15Z

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>
---
fs/nfs/localio.c | 11 +++--------
fs/nfs_common/nfslocalio.c | 39 +++++++++++++++++---------------------
2 files changed, 20 insertions(+), 30 deletions(-)

diff --git a/fs/nfs/localio.c b/fs/nfs/localio.c
index 5c21caeae075..3674dd86f095 100644
--- a/fs/nfs/localio.c
+++ b/fs/nfs/localio.c
@@ -279,14 +279,9 @@ nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred,
if (IS_ERR(new))
return NULL;
/* 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);
rcu_read_lock();
}
nf = nfs_local_file_get(nf);
diff --git a/fs/nfs_common/nfslocalio.c b/fs/nfs_common/nfslocalio.c
index 6a0bdea6d644..bdf251332b6b 100644
--- 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);

--
2.49.0