nfsd: cancel nfsd_shrinker_work using sync mode in nfs4_state_shutdown_net

In the normal case, when we excute `echo 0 > /proc/fs/nfsd/threads`, the
function `nfs4_state_destroy_net` in `nfs4_state_shutdown_net` will
release all resources related to the hashed `nfs4_client`. If the
`nfsd_client_shrinker` is running concurrently, the `expire_client`
function will first unhash this client and then destroy it. This can
lead to the following warning. Additionally, numerous use-after-free
errors may occur as well.

nfsd_client_shrinker         echo 0 > /proc/fs/nfsd/threads

expire_client                nfsd_shutdown_net
  unhash_client                ...
                               nfs4_state_shutdown_net
                                 /* won't wait shrinker exit */
  /*                             cancel_work(&nn->nfsd_shrinker_work)
   * nfsd_file for this          /* won't destroy unhashed client1 */
   * client1 still alive         nfs4_state_destroy_net
   */

                               nfsd_file_cache_shutdown
                                 /* trigger warning */
                                 kmem_cache_destroy(nfsd_file_slab)
                                 kmem_cache_destroy(nfsd_file_mark_slab)
  /* release nfsd_file and mark */
  __destroy_client

====================================================================
BUG nfsd_file (Not tainted): Objects remaining in nfsd_file on
__kmem_cache_shutdown()
--------------------------------------------------------------------
CPU: 4 UID: 0 PID: 764 Comm: sh Not tainted 6.12.0-rc3+ #1

 dump_stack_lvl+0x53/0x70
 slab_err+0xb0/0xf0
 __kmem_cache_shutdown+0x15c/0x310
 kmem_cache_destroy+0x66/0x160
 nfsd_file_cache_shutdown+0xac/0x210 [nfsd]
 nfsd_destroy_serv+0x251/0x2a0 [nfsd]
 nfsd_svc+0x125/0x1e0 [nfsd]
 write_threads+0x16a/0x2a0 [nfsd]
 nfsctl_transaction_write+0x74/0xa0 [nfsd]
 vfs_write+0x1a5/0x6d0
 ksys_write+0xc1/0x160
 do_syscall_64+0x5f/0x170
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

====================================================================
BUG nfsd_file_mark (Tainted: G    B   W         ): Objects remaining
nfsd_file_mark on __kmem_cache_shutdown()
--------------------------------------------------------------------

 dump_stack_lvl+0x53/0x70
 slab_err+0xb0/0xf0
 __kmem_cache_shutdown+0x15c/0x310
 kmem_cache_destroy+0x66/0x160
 nfsd_file_cache_shutdown+0xc8/0x210 [nfsd]
 nfsd_destroy_serv+0x251/0x2a0 [nfsd]
 nfsd_svc+0x125/0x1e0 [nfsd]
 write_threads+0x16a/0x2a0 [nfsd]
 nfsctl_transaction_write+0x74/0xa0 [nfsd]
 vfs_write+0x1a5/0x6d0
 ksys_write+0xc1/0x160
 do_syscall_64+0x5f/0x170
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

To resolve this issue, cancel `nfsd_shrinker_work` using synchronous
mode in nfs4_state_shutdown_net.

Fixes: 7c24fa2 ("NFSD: replace delayed_work with work_struct for nfsd_client_shrinker")
Signed-off-by: Yang Erkun <yangerkun@huaweicloud.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: Yang Erkun

fs/nfsd/nfs4state.c

@@ -8716,7 +8716,7 @@ nfs4_state_shutdown_net(struct net *net)
 	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
 
 	shrinker_free(nn->nfsd_client_shrinker);
-	cancel_work(&nn->nfsd_shrinker_work);
+	cancel_work_sync(&nn->nfsd_shrinker_work);
 	cancel_delayed_work_sync(&nn->laundromat_work);
 	locks_end_grace(&nn->nfsd4_manager);