nfs4: resend LAYOUTGET when there is a race that changes the seqid

pnfs_layout_process will check the returned layout stateid against what
the kernel has in-core. If it turns out that the stateid we received is
older, then we should resend the LAYOUTGET instead of falling back to
MDS I/O.
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Cc: stable@vger.kernel.org # 3.18+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/pnfs.c

@@ -872,6 +872,12 @@ send_layoutget(struct pnfs_layout_hdr *lo,
 
 	dprintk("--> %s\n", __func__);
 
+	/*
+	 * Synchronously retrieve layout information from server and
+	 * store in lseg. If we race with a concurrent seqid morphing
+	 * op, then re-send the LAYOUTGET.
+	 */
+	do {
 		lgp = kzalloc(sizeof(*lgp), gfp_flags);
 		if (lgp == NULL)
 			return NULL;
@@ -895,10 +901,9 @@ send_layoutget(struct pnfs_layout_hdr *lo,
 		lgp->gfp_flags = gfp_flags;
 		lgp->cred = lo->plh_lc_cred;
 
-	/* Synchronously retrieve layout information from server and
-	 * store in lseg.
-	 */
 		lseg = nfs4_proc_layoutget(lgp, gfp_flags);
+	} while (lseg == ERR_PTR(-EAGAIN));
+
 	if (IS_ERR(lseg)) {
 		switch (PTR_ERR(lseg)) {
 		case -ENOMEM:
@@ -1687,6 +1692,7 @@ pnfs_layout_process(struct nfs4_layoutget *lgp)
 		/* existing state ID, make sure the sequence number matches. */
 		if (pnfs_layout_stateid_blocked(lo, &res->stateid)) {
 			dprintk("%s forget reply due to sequence\n", __func__);
+			status = -EAGAIN;
 			goto out_forget_reply;
 		}
 		pnfs_set_layout_stateid(lo, &res->stateid, false);