[PATCH 2/2] nfsd: drain inflight callbacks in probe_callback_sync

[Chris Mason <clm@meta.com>]

nfsd4_destroy_session() calls nfsd4_probe_callback_sync() to quiesce
the backchannel before nfsd4_put_session_locked() drops the last
se_ref and frees the session. nfsd4_probe_callback_sync() only
flushes cl_callback_wq, which drains the work_struct that submits
CB RPCs but returns before the rpciod-side completion tail
(rpc_call_done -> rpc_release -> nfsd41_destroy_cb) has run. A CB
RPC submitted just before teardown can therefore still be executing
on rpciod, with clp->cl_cb_session pointing at the session about to
be freed:

    CPU 0 (DESTROY_SESSION)              CPU 1 (rpciod)
    -----                                -----
    nfsd4_probe_callback_sync(clp)
      flush_workqueue(cl_callback_wq)    nfsd4_cb_sequence_done()
                                           reads clp->cl_cb_session
    nfsd4_put_session_locked(ses)
      free_session(ses)
      kfree(ses)
                                         nfsd41_destroy_cb()
                                           dec cl_cb_inflight

The se_ref gate in nfsd4_put_session_locked() does not close this
window: the backchannel dispatch path does not take a se_ref via
nfsd4_get_session_locked(), so se_ref can already be zero while a
CB RPC is still in flight against the session.

cl_cb_inflight, added by commit 2bbfed98a4d8 ("nfsd: Fix races
between nfsd4_cb_release() and nfsd4_shutdown_callback()"), is the
barrier that covers the full window: nfsd4_run_cb() bumps it before
queue_work() and nfsd41_destroy_cb() drops it from rpc_release,
after the last cl_cb_session dereference. nfsd4_shutdown_callback()
already calls nfsd41_cb_inflight_wait_complete() after its
flush_workqueue() for this reason; nfsd4_probe_callback_sync() was
not updated when cl_cb_inflight was introduced.

Fix by waiting on cl_cb_inflight in nfsd4_probe_callback_sync()
after the workqueue flush, so every queued CB RPC has reached its
rpc_release before the caller proceeds to free the session.

Fixes: 2bbfed98a4d82ac4 ("nfsd: Fix races between nfsd4_cb_release() and nfsd4_shutdown_callback()")
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
---
 fs/nfsd/nfs4callback.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index 8af2d0cc37c2..6cf5591651e4 100644
--- a/fs/nfsd/nfs4callback.c
+++ b/fs/nfsd/nfs4callback.c
@@ -1246,6 +1246,7 @@ void nfsd4_probe_callback_sync(struct nfs4_client *clp)
 {
 	nfsd4_probe_callback(clp);
 	flush_workqueue(clp->cl_callback_wq);
+	nfsd41_cb_inflight_wait_complete(clp);
 }
 
 void nfsd4_change_callback(struct nfs4_client *clp, struct nfs4_cb_conn *conn)
-- 
2.52.0