NFSv4/pNFS: Fix a race to wake on NFS_LAYOUT_DRAIN

We found a few different systems hung up in writeback waiting on the same
page lock, and one task waiting on the NFS_LAYOUT_DRAIN bit in
pnfs_update_layout(), however the pnfs_layout_hdr's plh_outstanding count
was zero.

It seems most likely that this is another race between the waiter and waker
similar to commit ed0172a ("SUNRPC: Fix a race to wake a sync task").
Fix it up by applying the advised barrier.

Fixes: 880265c ("pNFS: Avoid a live lock condition in pnfs_update_layout()")
Signed-off-by: Benjamin Coddington <[email protected]>
Signed-off-by: Anna Schumaker <[email protected]>

Author: bcodding-rh

fs/nfs/pnfs.c

@@ -2059,8 +2059,10 @@ static void nfs_layoutget_begin(struct pnfs_layout_hdr *lo)
 static void nfs_layoutget_end(struct pnfs_layout_hdr *lo)
 {
 	if (atomic_dec_and_test(&lo->plh_outstanding) &&
-	    test_and_clear_bit(NFS_LAYOUT_DRAIN, &lo->plh_flags))
+	    test_and_clear_bit(NFS_LAYOUT_DRAIN, &lo->plh_flags)) {
+		smp_mb__after_atomic();
 		wake_up_bit(&lo->plh_flags, NFS_LAYOUT_DRAIN);
+	}
 }
 
 static bool pnfs_is_first_layoutget(struct pnfs_layout_hdr *lo)