[PATCH v4 9/9] NFSD: Release the export reference when reaping open stateids

[Chuck Lever <cel@kernel.org>]

nfs4_put_stid() releases the svc_export tracked in
nfs4_stid.sc_export, but free_ol_stateid_reaplist() frees open and
lock stateids by calling ->sc_free() directly, bypassing that path.
An open stateid takes an sc_export reference in nfs4_open() and a
lock stateid takes its own in init_lock_stateid(); both reach
free_ol_stateid_reaplist() through their normal teardown, the open
stateid via release_open_stateid() and the lock stateid via
nfsd4_release_lockowner(), each through put_ol_stateid_locked().
The reference is therefore never dropped, pinning the export and
blocking unmount for the lifetime of the stateid.

Release sc_export in free_ol_stateid_reaplist() the way
nfs4_put_stid() does. ->sc_free() runs once per stateid, and a
stateid reaches free_ol_stateid_reaplist() or nfs4_put_stid() but
never both, so the reference is dropped exactly once. Revoked
stateids reach this path with sc_export already cleared by
drop_stid_export(), so they are skipped rather than double-freed.

nfs4_put_stid() itself read sc_export before acquiring cl_lock.
drop_stid_export() clears that field and releases the reference
under cl_lock, so a concurrent revocation could drop the export in
the window between the read and the final put, releasing the same
reference twice. Read sc_export while cl_lock is held so the two
paths serialize and the reference is released exactly once.

Fixes: ba0cde5dc81d ("NFSD: Track svc_export in nfs4_stid")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260707-cel-v3-0-7c0cc16fd54f@kernel.org?part=9
Signed-off-by: Chuck Lever <cel@kernel.org>
---
 fs/nfsd/nfs4state.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 20556b8f186a..e988dfebf75e 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1272,9 +1272,9 @@ alloc_init_dir_deleg(struct nfs4_client *clp, struct nfs4_file *fp)
 void
 nfs4_put_stid(struct nfs4_stid *s)
 {
-	struct svc_export *exp = s->sc_export;
 	struct nfs4_file *fp = s->sc_file;
 	struct nfs4_client *clp = s->sc_client;
+	struct svc_export *exp;
 
 	might_lock(&clp->cl_lock);
 
@@ -1285,6 +1285,8 @@ nfs4_put_stid(struct nfs4_stid *s)
 	idr_remove(&clp->cl_stateids, s->sc_stateid.si_opaque.so_id);
 	if (s->sc_status & SC_STATUS_ADMIN_REVOKED)
 		atomic_dec(&s->sc_client->cl_admin_revoked);
+	/* Read under cl_lock to serialize with drop_stid_export(). */
+	exp = s->sc_export;
 	nfs4_free_cpntf_statelist(clp->net, s);
 	spin_unlock(&clp->cl_lock);
 	s->sc_free(s);
@@ -1744,6 +1746,7 @@ static void
 free_ol_stateid_reaplist(struct list_head *reaplist)
 {
 	struct nfs4_ol_stateid *stp;
+	struct svc_export *exp;
 	struct nfs4_file *fp;
 
 	might_sleep();
@@ -1753,9 +1756,12 @@ free_ol_stateid_reaplist(struct list_head *reaplist)
 				       st_locks);
 		list_del(&stp->st_locks);
 		fp = stp->st_stid.sc_file;
+		exp = stp->st_stid.sc_export;
 		stp->st_stid.sc_free(&stp->st_stid);
 		if (fp)
 			put_nfs4_file(fp);
+		if (exp)
+			exp_put(exp);
 	}
 }
 

-- 
2.54.0