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

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);

sys/kern/vfs_bio.c

@@ -758,11 +758,15 @@ bwrite(struct buf * bp)
 		int rtval = bufwait(bp);
 		brelse(bp);
 		return (rtval);
-	} else {
+	} else if ((oldflags & B_NOWDRAIN) == 0) {
 		/*
 		 * don't allow the async write to saturate the I/O
-		 * system.  There is no chance of deadlock here because
-		 * we are blocking on I/O that is already in-progress.
+		 * system.  Deadlocks can occur only if a device strategy
+		 * routine (like in MD) turns around and issues another
+		 * high-level write, in which case B_NOWDRAIN is expected
+		 * to be set.  Otherwise we will not deadlock here because
+		 * we are blocking waiting for I/O that is already in-progress
+		 * to complete.
 		 */
 		waitrunningbufspace();
 	}
@@ -1286,7 +1290,8 @@ brelse(struct buf * bp)
 
 	/* unlock */
 	BUF_UNLOCK(bp);
-	bp->b_flags &= ~(B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF | B_DIRECT);
+	bp->b_flags &= ~(B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF | 
+			B_DIRECT | B_NOWDRAIN);
 	bp->b_ioflags &= ~BIO_ORDERED;
 	if ((bp->b_flags & B_DELWRI) == 0 && (bp->b_xflags & BX_VNDIRTY))
 		panic("brelse: not dirty");

sys/kern/vfs_cluster.c

@@ -836,7 +836,7 @@ cluster_wbuild(vp, size, start_lbn, len)
 		bp->b_data = (char *)((vm_offset_t)bp->b_data |
 		    ((vm_offset_t)tbp->b_data & PAGE_MASK));
 		bp->b_flags |= B_CLUSTER |
-				(tbp->b_flags & (B_VMIO | B_NEEDCOMMIT));
+				(tbp->b_flags & (B_VMIO | B_NEEDCOMMIT | B_NOWDRAIN));
 		bp->b_iodone = cluster_callback;
 		pbgetvp(vp, bp);
 		/*

sys/nfsclient/nfs_bio.c

@@ -961,6 +961,12 @@ nfs_write(struct vop_write_args *ap)
 			}
 			vfs_bio_set_validclean(bp, on, n);
 		}
+		/*
+		 * If IO_NOWDRAIN then set B_NOWDRAIN (nfs-backed MD 
+		 * filesystem)
+		 */
+		if (ioflag & IO_NOWDRAIN)
+			bp->b_flags |= B_NOWDRAIN;
 
 		/*
 		 * If IO_SYNC do bwrite().

sys/sys/buf.h

@@ -192,6 +192,11 @@ struct buf {
  *			the pages underlying the buffer.  B_DIRECT is
  *			sticky until the buffer is released and typically
  *			only has an effect when B_RELBUF is also set.
+ *
+ *	B_NOWDRAIN	This flag should be set when a device (like MD)
+ *			does a turn-around VOP_WRITE from its strategy
+ *			routine.  This flag prevents bwrite() from blocking
+ *			in wdrain, avoiding a deadlock situation.
  */
 
 #define	B_AGE		0x00000001	/* Move to age queue when I/O done. */
@@ -204,7 +209,7 @@ struct buf {
 #define	B_DELWRI	0x00000080	/* Delay I/O until buffer reused. */
 #define	B_DONE		0x00000200	/* I/O completed. */
 #define	B_EINTR		0x00000400	/* I/O was interrupted */
-#define	B_00000800	0x00000800	/* Available flag. */
+#define	B_NOWDRAIN	0x00000800	/* Avoid wdrain deadlock */
 #define	B_SCANNED	0x00001000	/* VOP_FSYNC funcs mark written bufs */
 #define	B_INVAL		0x00002000	/* Does not contain valid info. */
 #define	B_LOCKED	0x00004000	/* Locked in core (not reusable). */

sys/sys/vnode.h

@@ -222,6 +222,7 @@ struct vattr {
 #define	IO_INVAL	0x40		/* invalidate after I/O */
 #define	IO_ASYNC	0x80		/* bawrite rather then bdwrite */
 #define IO_DIRECT	0x100		/* attempt to bypass buffer cache */
+#define IO_NOWDRAIN	0x200		/* do not block on wdrain */
 
 /*
  *  Modes.  Some values same as Ixxx entries from inode.h for now.

sys/ufs/ufs/ufs_readwrite.c

@@ -511,6 +511,8 @@ WRITE(ap)
 			break;
 		if (ioflag & IO_DIRECT)
 			bp->b_flags |= B_DIRECT;
+		if (ioflag & IO_NOWDRAIN)
+			bp->b_flags |= B_NOWDRAIN;
 
 		if (uio->uio_offset + xfersize > ip->i_size) {
 			ip->i_size = uio->uio_offset + xfersize;