NFS/filelayout: Fix racy setting of fl->dsaddr in filelayout_check_deviceid()

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>

1 files changed, 11 insertions, 2 deletions

diff --git a/fs/nfs/filelayout/filelayout.c b/fs/nfs/filelayout/filelayout.c
index 080fc6b278bd..44c638b7876c 100644
--- a/fs/nfs/filelayout/filelayout.c
+++ b/fs/nfs/filelayout/filelayout.c
@@ -542,6 +542,10 @@ filelayout_check_deviceid(struct pnfs_layout_hdr *lo,
         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);
@@ -553,8 +557,6 @@ filelayout_check_deviceid(struct pnfs_layout_hdr *lo,
         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);
@@ -570,6 +572,13 @@ filelayout_check_deviceid(struct pnfs_layout_hdr *lo,
                 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: