[PATCH 5/6] nfs_localio: protect race between nfs_uuid_put() and nfs_close_local_fh()

NeilBrown
2025-05-09T00:49:16Z

nfs_uuid_put() and nfs_close_local_fh() can race if a "struct
nfs_file_localio" is released at the same time that nfsd calls
nfs_localio_invalidate_clients().

It is important that neither of these functions completes after the
other has started looking at a given nfs_file_localio and before it
finishes.

If nfs_uuid_put() exits while nfs_close_local_fh() is closing ro_file
and rw_file it could return to __nfd_file_cache_purge() while some files
are still referenced so the purge may not succeed.

If nfs_close_local_fh() exits while nfsd_uuid_put() is still closing the
files then the "struct nfs_file_localio" could be freed while
nfsd_uuid_put() is still looking at it. This side is currently handled
by copying the pointers out of ro_file and rw_file before deleting from
the list in nfsd_uuid. We need to preserve this while ensuring that
nfsd_uuid_put() does wait for nfs_close_local_fh().

This patch use nfl->uuid and nfl->list to provide the required
interlock.

nfs_uuid_put() removes the nfs_file_localio from the list, then drops
locks and puts the two files, then reclaims the spinlock and sets
->nfs_uuid to NULL.

nfs_close_local_fh() operates in the reverse order, setting ->nfs_uuid
to NULL, then closing the files, then unlinking from the list.

If nfs_uuid_put() finds that ->nfs_uuid is already NULL, it waits for
the nfs_file_localio to be removed from the list. If
nfs_close_local_fh() find that it has already been unlinked it waits for
->nfs_uuid to become NULL. This ensure that one of the two tries to
close the files, but they each waits for the other.

As nfs_uuid_put() is making the list empty, change from a
list_for_each_safe loop to a while that always takes the first entry.
This makes the intent more clear.
Also don't move the list to a temporary local list as this would defeat
the guarantees required for the interlock.

Fixes: 86e00412254a ("nfs: cache all open LOCALIO nfsd_file(s) in client")
Signed-off-by: NeilBrown <neil@brown.name>
---
fs/nfs_common/nfslocalio.c | 81 ++++++++++++++++++++++++++------------
1 file changed, 56 insertions(+), 25 deletions(-)

diff --git a/fs/nfs_common/nfslocalio.c b/fs/nfs_common/nfslocalio.c
index 49c59f0c78c6..1dd5a8cca064 100644
--- a/fs/nfs_common/nfslocalio.c
+++ b/fs/nfs_common/nfslocalio.c
@@ -151,8 +151,7 @@ EXPORT_SYMBOL_GPL(nfs_localio_enable_client);
*/
static bool nfs_uuid_put(nfs_uuid_t *nfs_uuid)
{
- LIST_HEAD(local_files);
- struct nfs_file_localio *nfl, *tmp;
+ struct nfs_file_localio *nfl;

spin_lock(&nfs_uuid->lock);
if (unlikely(!rcu_access_pointer(nfs_uuid->net))) {
@@ -166,37 +165,49 @@ static bool nfs_uuid_put(nfs_uuid_t *nfs_uuid)
nfs_uuid->dom = NULL;
}

- list_splice_init(&nfs_uuid->files, &local_files);
- spin_unlock(&nfs_uuid->lock);
-
/* Walk list of files and ensure their last references dropped */
- list_for_each_entry_safe(nfl, tmp, &local_files, list) {
+
+ while ((nfl = list_first_entry_or_null(&nfs_uuid->files,
+ struct nfs_file_localio,
+ list)) != NULL) {
struct nfsd_file *ro_nf;
struct nfsd_file *rw_nf;

+ /* If nfs_uuid is already NULL, nfs_close_local_fh is
+ * closing and we must wait, else we unlink and close.
+ */
+ if (rcu_access_pointer(nfl->nfs_uuid) == NULL) {
+ /* nfs_close_local_fh() is doing the
+ * close and we must wait. until it unlinks
+ */
+ wait_var_event_spinlock(nfl,
+ list_first_entry_or_null(
+ &nfs_uuid->files,
+ struct nfs_file_localio,
+ list) != nfl,
+ &nfs_uuid->lock);
+ continue;
+ }
+
ro_nf = unrcu_pointer(xchg(&nfl->ro_file, NULL));
rw_nf = unrcu_pointer(xchg(&nfl->rw_file, NULL));

- spin_lock(&nfs_uuid->lock);
/* Remove nfl from nfs_uuid->files list */
list_del_init(&nfl->list);
spin_unlock(&nfs_uuid->lock);
- /* 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);
-
cond_resched();
+ spin_lock(&nfs_uuid->lock);
+ /* Now we can allow racing nfs_close_local_fh() to
+ * skip the locking.
+ */
+ RCU_INIT_POINTER(nfl->nfs_uuid, NULL);
+ wake_up_var_locked(&nfl->nfs_uuid, &nfs_uuid->lock);
}

- spin_lock(&nfs_uuid->lock);
- BUG_ON(!list_empty(&nfs_uuid->files));
-
/* Remove client from nn->local_clients */
if (nfs_uuid->list_lock) {
spin_lock(nfs_uuid->list_lock);
@@ -304,23 +315,43 @@ void nfs_close_local_fh(struct nfs_file_localio *nfl)
return;
}

- ro_nf = unrcu_pointer(xchg(&nfl->ro_file, NULL));
- rw_nf = unrcu_pointer(xchg(&nfl->rw_file, NULL));
-
spin_lock(&nfs_uuid->lock);
- /* Remove nfl from nfs_uuid->files list */
- list_del_init(&nfl->list);
+ if (!rcu_access_pointer(nfl->nfs_uuid)) {
+ /* nfs_uuid_put has finished here */
+ spin_unlock(&nfs_uuid->lock);
+ rcu_read_unlock();
+ return;
+ }
+ if (list_empty(&nfs_uuid->files)) {
+ /* nfs_uuid_put() has started closing files, wait for it
+ * to finished
+ */
+ spin_unlock(&nfs_uuid->lock);
+ rcu_read_unlock();
+ wait_var_event(&nfl->nfs_uuid,
+ rcu_access_pointer(nfl->nfs_uuid) == NULL);
+ return;
+ }
+ /* tell nfs_uuid_put() to wait for us */
+ RCU_INIT_POINTER(nfl->nfs_uuid, NULL);
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);

+ 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);
+
+ /* Remove nfl from nfs_uuid->files list and signal nfs_uuid_put()
+ * that we are done. The moment we drop the spinlock the
+ * nfs_uuid could be freed.
+ */
+ spin_lock(&nfs_uuid->lock);
+ list_del_init(&nfl->list);
+ wake_up_var_locked(&nfl->nfs_uuid, &nfs_uuid->lock);
+ spin_unlock(&nfs_uuid->lock);
}
EXPORT_SYMBOL_GPL(nfs_close_local_fh);

--
2.49.0