Implement IO_NOWDRAIN and B_NOWDRAIN - prevents the buffer cache from blocking

in wdrain during a write.  This flag needs to be used in devices whos
strategy routines turn-around and issue another high level I/O, such as
when MD turns around and issues a VOP_WRITE to vnode backing store, in order
to avoid deadlocking the dirty buffer draining code.

Remove a vprintf() warning from MD when the backing vnode is found to be
in-use.  The syncer of buf_daemon could be flushing the backing vnode at
the time of an MD operation so the warning is not correct.

MFC after:	1 week

1 files changed, 7 insertions, 4 deletions

diff --git a/sys/dev/md/md.c b/sys/dev/md/md.c
index 0a69769..bb4b7f0 100644
--- a/sys/dev/md/md.c
+++ b/sys/dev/md/md.c
@@ -386,15 +386,18 @@ mdstart_vnode(struct md_s *sc)
 			auio.uio_rw = UIO_WRITE;
 		auio.uio_resid = bp->bio_bcount;
 		auio.uio_td = curthread;
-		if (VOP_ISLOCKED(sc->vnode, NULL))
-			vprint("unexpected md driver lock", sc->vnode);
+		/*
+		 * When reading set IO_DIRECT to try to avoid double-caching
+		 * the data.  When writing IO_DIRECT is not optimal, but we
+		 * must set IO_NOWDRAIN to avoid a wdrain deadlock.
+		 */
 		if (bp->bio_cmd == BIO_READ) {
 			vn_lock(sc->vnode, LK_EXCLUSIVE | LK_RETRY, curthread);
-			error = VOP_READ(sc->vnode, &auio, 0, sc->cred);
+			error = VOP_READ(sc->vnode, &auio, IO_DIRECT, sc->cred);
 		} else {
 			(void) vn_start_write(sc->vnode, &mp, V_WAIT);
 			vn_lock(sc->vnode, LK_EXCLUSIVE | LK_RETRY, curthread);
-			error = VOP_WRITE(sc->vnode, &auio, 0, sc->cred);
+			error = VOP_WRITE(sc->vnode, &auio, IO_NOWDRAIN, sc->cred);
 			vn_finished_write(mp);
 		}
 		VOP_UNLOCK(sc->vnode, 0, curthread);