From: Trond Myklebust <trond.myklebust@primarydata.com>
Date: Thu, 20 Jul 2017 17:00:02 -0400
Subject: [PATCH] NFS/filelayout: Fix racy setting of fl->dsaddr in
 filelayout_check_deviceid()
Git-commit: 1ebf980127924c639e2b85c08468311ba1c95b70
Patch-mainline: v4.13-rc2
References: bsc#1105190

We must set fl->dsaddr once, and once only, even if there are multiple
processes calling filelayout_check_deviceid() for the same layout
segment.

Reported-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Acked-by: NeilBrown <neilb@suse.com>

---
 fs/nfs/filelayout/filelayout.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

--- a/fs/nfs/filelayout/filelayout.c
+++ b/fs/nfs/filelayout/filelayout.c
@@ -570,6 +570,10 @@ filelayout_check_deviceid(struct pnfs_la
 	struct nfs4_file_layout_dsaddr *dsaddr;
 	int status = -EINVAL;
 
+	/* Is the deviceid already set? If so, we're good. */
+	if (fl->dsaddr != NULL)
+		return 0;
+
 	/* find and reference the deviceid */
 	d = nfs4_find_get_deviceid(NFS_SERVER(lo->plh_inode), &fl->deviceid,
 			lo->plh_lc_cred, gfp_flags);
@@ -581,8 +585,6 @@ filelayout_check_deviceid(struct pnfs_la
 	if (filelayout_test_devid_unavailable(&dsaddr->id_node))
 		goto out_put;
 
-	fl->dsaddr = dsaddr;
-
 	if (fl->first_stripe_index >= dsaddr->stripe_count) {
 		dprintk("%s Bad first_stripe_index %u\n",
 				__func__, fl->first_stripe_index);
@@ -598,6 +600,13 @@ filelayout_check_deviceid(struct pnfs_la
 		goto out_put;
 	}
 	status = 0;
+
+	/*
+	 * Atomic compare and xchange to ensure we don't scribble
+	 * over a non-NULL pointer.
+	 */
+	if (cmpxchg(&fl->dsaddr, NULL, dsaddr) != NULL)
+		goto out_put;
 out:
 	return status;
 out_put: