nfsd: document callback_wq serialization of callback code
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  • nfsd: document callback_wq serialization of callback code
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Commit Message Nov. 8, 2019, 5:54 p.m. UTC
From: "J. Bruce Fields" <>
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 <>
Signed-off-by: J. Bruce Fields <>
 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.

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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;