[PATCH] nfsd: document callback_wq serialization of callback code

["J. Bruce Fields" <bfields@fieldses.org>]

From: "J. Bruce Fields" <bfields@redhat.com>
Date: Tue, 29 Oct 2019 16:02:18 -0400

The callback code relies on the fact that much of it is only ever called
from the ordered workqueue callback_wq, and this is worth documenting.

Reported-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
---
 fs/nfsd/nfs4callback.c | 6 ++++++
 1 file changed, 6 insertions(+)

Also adding a comment, since I know this was a source of confusion when
investigating these races.

diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index c94768b096a3..24534db87e86 100644
--- a/fs/nfsd/nfs4callback.c
+++ b/fs/nfsd/nfs4callback.c
@@ -1243,6 +1243,12 @@ static struct nfsd4_conn * __nfsd4_find_backchannel(struct nfs4_client *clp)
 	return NULL;
 }
 
+/*
+ * Note there isn't a lot of locking in this code; instead we depend on
+ * the fact that it is run from the callback_wq, which won't run two
+ * work items at once.  So, for example, callback_wq handles all access
+ * of cl_cb_client and all calls to rpc_create or rpc_shutdown_client.
+ */
 static void nfsd4_process_cb_update(struct nfsd4_callback *cb)
 {
 	struct nfs4_cb_conn conn;
-- 
2.23.0