ocfs2/dlm: drain join handlers before releasing module text

The code for a registered DLM handler must remain resident until its
last invocation finishes. dlm_unregister_net_handlers() only removes
dlm_join_handlers from the o2net registry. o2net_handler_get() takes a
kref on the wrapper, but that reference does not pin ocfs2_dlm text.
o2net_process_message() calls nh_func after dropping o2net_handler_lock.

With CONFIG_OCFS2_FS_O2CB=m, ocfs2_dlm can be loaded independently of
ocfs2_stack_o2cb while nodemanager continues handling messages from a
configured, heartbeating peer. If no dependent module or module reference
prevents normal unload, the following ordering is possible:

  Receive worker                       Module unload
  --------------                       -------------
  o2net_handler_get()
    take wrapper kref
    drop handler lock
  <preempted>
                                       dlm_exit()
                                         unregister handlers
                                       release DLM module text
  call saved nh_func
    in released module text

Use o2net_unregister_and_flush_handler_list() in the shared DLM
unregister helper. It first prevents new lookups, then waits for the
ordered o2net workqueue to drain, including a concurrent destruction of
that queue. The acquired invocation therefore finishes before dlm_exit()
returns and the loader releases the callback code. The same helper
covers partial registration and dlm_init() error unwind before module
memory is released.

Skip the drain when the private join-handler list is empty. Early
dlm_init() failures can reach this helper before any handler has been
published. Partial-registration failure also reaches it a second time
after the first call has already unregistered and drained the list.
Neither case needs to take o2net_wq_mutex or drain unrelated work.
The list is changed only by serialized module init/exit paths; a
nonempty list still uses the full unregister-and-flush helper.

Oops report as below:

    Oops: int3: 0000 [#1] SMP KASAN NOPTI
    CPU: 2 UID: 0 PID: 54 Comm: kworker/u16:3 Not tainted 7.3.0-rc4-next-20260921-pmb-ocfs2-functional-v1+ #1 PREEMPT(lazy)

Hardware identification omitted.

    Workqueue: o2net o2net_rx_until_empty [ocfs2_nodemanager]
    RIP: 0010:0xffffffffc0423871
    [Instruction byte dump omitted.]

Register dump omitted.

    Call Trace:
     <TASK>
     ? o2net_rx_until_empty+0x19b1/0x3300 [ocfs2_nodemanager]
     ? __pfx_o2net_rx_until_empty+0x10/0x10 [ocfs2_nodemanager]
     ? lock_acquire+0x190/0x300
     ? process_one_work+0x935/0x1b40
     ? process_one_work+0x834/0x1b40
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? lock_release+0xc8/0x290
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? process_one_work+0x9a8/0x1b40
     ? __pfx_process_one_work+0x10/0x10
     ? lock_acquire+0x190/0x300
     ? lock_is_held_type+0x8f/0x100
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? worker_thread+0x65c/0xe40
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? __kthread_parkme+0x177/0x220
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? __pfx_worker_thread+0x10/0x10
     ? kthread+0x351/0x460
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? __pfx_kthread+0x10/0x10
     ? ret_from_fork+0x659/0x940
     ? __pfx_ret_from_fork+0x10/0x10
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? __switch_to+0x74f/0xf70
     ? __pfx_kthread+0x10/0x10
     ? ret_from_fork_asm+0x1a/0x30
     </TASK>
    Modules linked in: ocfs2_nodemanager [last unloaded: ocfs2_dlm]
    ---[ end trace 0000000000000000 ]---

Link: https://lore.kernel.org/pm-ocfs2-objects-candidate-0076-v6-12de5f4628f57fba3be3@gmail.com
Fixes: 6714d8e86b ("[PATCH] OCFS2: The Second Oracle Cluster Filesystem")
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Assisted-by: LLM
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Heming Zhao <heming.zhao@suse.com>
Cc: Jia-Ju Bai <baijiaju1990@gmail.com>
Cc: <jjzuming@gmail.com>
Cc: <stable@vger.kernel.org>

fs/ocfs2/dlm/dlmdomain.c

@@ -2215,7 +2215,8 @@ static LIST_HEAD(dlm_join_handlers);
 
 static void dlm_unregister_net_handlers(void)
 {
-	o2net_unregister_handler_list(&dlm_join_handlers);
+	if (!list_empty(&dlm_join_handlers))
+		o2net_unregister_and_flush_handler_list(&dlm_join_handlers);
 }
 
 static int dlm_register_net_handlers(void)