[PATCH] NFSv4: Fix deadlock during the running of state manager

[Li Lingfeng <lilingfeng3@huawei.com>]

Unlinking file may cause the following deadlock in state manager:
[root@localhost test]# cat /proc/2943/stack
[<0>] rpc_wait_bit_killable+0x1a/0x90
[<0>] _nfs4_proc_delegreturn+0x60f/0x760
[<0>] nfs4_proc_delegreturn+0x13d/0x2a0
[<0>] nfs_do_return_delegation+0xba/0x110
[<0>] nfs_end_delegation_return+0x32c/0x620
[<0>] nfs_complete_unlink+0xc7/0x290
[<0>] nfs_dentry_iput+0x36/0x50
[<0>] __dentry_kill+0xaa/0x250
[<0>] dput.part.0+0x26c/0x4d0
[<0>] __put_nfs_open_context+0x1d9/0x260
[<0>] nfs4_open_reclaim+0x77/0xa0
[<0>] nfs4_do_reclaim+0x385/0xf40
[<0>] nfs4_state_manager+0x762/0x14e0
[<0>] nfs4_run_state_manager+0x181/0x330
[<0>] kthread+0x1a7/0x1f0
[<0>] ret_from_fork+0x34/0x60
[<0>] ret_from_fork_asm+0x1a/0x30
[root@localhost test]#

It can be reproduced by following steps:
1) client: open file
2) client: unlink file
3) server: service restart(trigger state manager in client)
4) client: close file(in nfs4_open_reclaim, between nfs4_do_open_reclaim
and put_nfs_open_context)

Since the file has been open, unlinking will just set DCACHE_NFSFS_RENAMED
for the dentry like this:
nfs_unlink
 nfs_sillyrename
  nfs_async_unlink
   // set DCACHE_NFSFS_RENAMED

Restarting service will trigger state manager in client.
(1) NFS4_SLOT_TBL_DRAINING will be set to nfs4_slot_table since session
has been reset.
(2) DCACHE_NFSFS_RENAMED is detected in nfs_dentry_iput. Therefore,
nfs_complete_unlink is called to trigger delegation return.
(3) Due to the slot table being in draining state and sa_privileged being
0, the delegation return will be queued and wait.
nfs4_state_manager
 nfs4_reset_session
  nfs4_begin_drain_session
   nfs4_drain_slot_tbl
    // set NFS4_SLOT_TBL_DRAINING (1)
 nfs4_do_reclaim
  nfs4_open_reclaim
   __put_nfs_open_context
    __dentry_kill
     nfs_dentry_iput // check DCACHE_NFSFS_RENAMED (2)
      nfs_complete_unlink
       nfs_end_delegation_return
        nfs_do_return_delegation
         nfs4_proc_delegreturn
          _nfs4_proc_delegreturn
           rpc_run_task
            ...
            nfs4_delegreturn_prepare
             nfs4_setup_sequence
              nfs4_slot_tbl_draining // check NFS4_SLOT_TBL_DRAINING
                                     // and sa_privileged is 0 (3)
               rpc_sleep_on // set queued and add to slot_tbl_waitq
                // rpc_task is async and wait in __rpc_execute
           rpc_wait_for_completion_task
            __rpc_wait_for_completion_task
             out_of_line_wait_on_bit
              rpc_wait_bit_killable // wait for rpc_task to complete
 <-------- can not get here to wake up rpc_task -------->
 nfs4_end_drain_session
  nfs4_end_drain_slot_table
   nfs41_wake_slot_table

On the one hand, the state manager is blocked by the unfinished delegation
return. As a result, nfs4_end_drain_session cannot be invoked to clear
NFS4_SLOT_TBL_DRAINING and wake up waiting tasks.
On the other hand, since NFS4_SLOT_TBL_DRAINING is not cleared,
delegation return can only wait in the queue, resulting in a deadlock.

Fix it by turning the delegation return into a privileged operation for
the case where the nfs_client is in NFS4CLNT_RECLAIM_REBOOT state.

Fixes: 977fcc2b0b41 ("NFS: Add a delegation return into nfs4_proc_unlink_setup()")
Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
---
 fs/nfs/nfs4proc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 405f17e6e0b4..f3b1f2725c4e 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -6878,7 +6878,7 @@ static int _nfs4_proc_delegreturn(struct inode *inode, const struct cred *cred,
 		data->res.sattr_res = true;
 	}
 
-	if (!data->inode)
+	if (!data->inode || test_bit(NFS4CLNT_RECLAIM_REBOOT, &server->nfs_client->cl_state))
 		nfs4_init_sequence(&data->args.seq_args, &data->res.seq_res, 1,
 				   1);
 	else
-- 
2.31.1