NFS: add comments clarifying the use of nfs_post_op_update()

Comments-only change to clarify a detail of the NFS protocol and how it is
implemented in Linux.

Test plan:
None.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 38ba5c09af08..c12effb46fe5 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -1253,7 +1253,13 @@ int nfs_writeback_done(struct rpc_task *task, struct nfs_write_data *data)
         dprintk("NFS: %4d nfs_writeback_done (status %d)\n",
                 task->tk_pid, task->tk_status);
 
-        /* Call the NFS version-specific code */
+        /*
+         * ->write_done will attempt to use post-op attributes to detect
+         * conflicting writes by other clients.  A strict interpretation
+         * of close-to-open would allow us to continue caching even if
+         * another writer had changed the file, but some applications
+         * depend on tighter cache coherency when writing.
+         */
         status = NFS_PROTO(data->inode)->write_done(task, data);
         if (status != 0)
                 return status;