[PATCH v2 21/21] nfsd: drop the stateid, not the stateowner, on seqid_op replay retry

[Jeff Layton <jlayton@kernel.org>]

In nfs4_preprocess_seqid_op() the stateid is obtained from
nfsd4_lookup_stateid(), which holds a reference on the nfs4_stid
(sc_count) but takes no reference on the stateowner. openlockstateid()
merely casts that stid and likewise takes no reference.

When nfsd4_cstate_assign_replay() returns -EAGAIN (the replay owner is
being torn down, RP_UNHASHED) it has not taken a stateowner reference on
that path. The error handling nevertheless called
nfs4_put_stateowner(stp->st_stateowner), dropping an so_count reference
the function never acquired -- risking a stateowner refcount underflow and
use-after-free -- while leaking the sc_count reference held on the stid.
The leaked stid reference can also stall a concurrent
nfsd4_close_open_stateid() waiting for sc_count to drop.

Drop the reference actually held -- the stid -- before retrying. The
stateowner stays alive through the reference held by the stid. This mirrors
the open path in nfsd4_process_open2(), where the put balances a reference
that path explicitly holds on the stateowner.

Fixes: eec762080008 ("nfsd: replace rp_mutex to avoid deadlock in move_to_close_lru()")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/nfsd/nfs4state.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index a0c97bff3cff..bef0ec9be459 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -7876,7 +7876,7 @@ nfs4_preprocess_seqid_op(struct nfsd4_compound_state *cstate, u32 seqid,
 		return status;
 	stp = openlockstateid(s);
 	if (nfsd4_cstate_assign_replay(cstate, stp->st_stateowner) == -EAGAIN) {
-		nfs4_put_stateowner(stp->st_stateowner);
+		nfs4_put_stid(&stp->st_stid);
 		goto retry;
 	}
 

-- 
2.54.0