[RFC PATCH] nfsd: convert the nfsd_users to atomic_t

[chenxiaosong@chenxiaosong.com]

From: ChenXiaoSong <chenxiaosong@kylinos.cn>

Before commit 38f080f3cd19 ("NFSD: Move callback_wq into struct nfs4_client"),
we had a null-ptr-deref in nfsd4_probe_callback() (Link[1]):

 nfsd: last server has exited, flushing export cache
 NFSD: starting 90-second grace period (net f0000030)
 Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
 ...
 Call trace:
  __queue_work+0xb4/0x558
  queue_work_on+0x88/0x90
  nfsd4_probe_callback+0x4c/0x58 [nfsd]
 NFSD: starting 90-second grace period (net f0000030)
  nfsd4_probe_callback_sync+0x20/0x38 [nfsd]
  nfsd4_init_conn.isra.57+0x8c/0xa8 [nfsd]
  nfsd4_create_session+0x5b8/0x718 [nfsd]
  nfsd4_proc_compound+0x4c0/0x710 [nfsd]
  nfsd_dispatch+0x104/0x248 [nfsd]
  svc_process_common+0x348/0x808 [sunrpc]
  svc_process+0xb0/0xc8 [sunrpc]
  nfsd+0xf0/0x160 [nfsd]
  kthread+0x134/0x138
  ret_from_fork+0x10/0x18
 Code: aa1c03e0 97ffffba aa0003e2 b5000780 (f9400262)
 SMP: stopping secondary CPUs
 Starting crashdump kernel...
 Bye!

One of the cases is:

    task A (cpu 1)    |   task B (cpu 2)     |   task C (cpu 3)
 ---------------------|----------------------|---------------------------------
 nfsd_startup_generic | nfsd_startup_generic |
   nfsd_users == 0    |  nfsd_users == 0     |
   nfsd_users++       |  nfsd_users++        |
   nfsd_users == 1    |                      |
   ...                |                      |
   callback_wq == xxx |                      |
 ---------------------|----------------------|---------------------------------
                      |                      | nfsd_shutdown_generic
                      |                      |   nfsd_users == 1
                      |                      |   --nfsd_users
                      |                      |   nfsd_users == 0
                      |                      |   ...
                      |                      |   callback_wq == xxx
                      |                      |   destroy_workqueue(callback_wq)
 ---------------------|----------------------|---------------------------------
                      |  nfsd_users == 1     |
                      |  ...                 |
                      |  callback_wq == yyy  |

After commit 38f080f3cd19 ("NFSD: Move callback_wq into struct nfs4_client"),
this issue no longer occurs, but we should still convert the nfsd_users
to atomic_t to prevent other similar issues.

Link[1]: https://chenxiaosong.com/en/nfs/en-null-ptr-deref-in-nfsd4_probe_callback.html
Co-developed-by: huhai <huhai@kylinos.cn>
Signed-off-by: huhai <huhai@kylinos.cn>
Signed-off-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
---
 fs/nfsd/nfssvc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index 9b3d6cff0e1e..08b1f9ebdc2a 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -270,13 +270,13 @@ static int nfsd_init_socks(struct net *net, const struct cred *cred)
 	return 0;
 }
 
-static int nfsd_users = 0;
+static atomic_t nfsd_users = ATOMIC_INIT(0);
 
 static int nfsd_startup_generic(void)
 {
 	int ret;
 
-	if (nfsd_users++)
+	if (atomic_fetch_inc(&nfsd_users))
 		return 0;
 
 	ret = nfsd_file_cache_init();
@@ -291,13 +291,13 @@ static int nfsd_startup_generic(void)
 out_file_cache:
 	nfsd_file_cache_shutdown();
 dec_users:
-	nfsd_users--;
+	atomic_dec(&nfsd_users);
 	return ret;
 }
 
 static void nfsd_shutdown_generic(void)
 {
-	if (--nfsd_users)
+	if (atomic_dec_return(&nfsd_users))
 		return;
 
 	nfs4_state_shutdown();
-- 
2.34.1