This update eliminates a lock-order reversal warning discovered

whle tracking down the system hang reported in kern/160662 and
corrected in revision 225806. The LOR is not the cause of the system
hang and indeed cannot cause an actual deadlock. However, it can
be easily eliminated by defering the acquisition of a buflock until
after all the vnode locks have been acquired.

Reported by:     Hans Ottevanger
PR:              kern/160662

1 files changed, 24 insertions, 21 deletions

diff --git a/sys/ufs/ffs/ffs_snapshot.c b/sys/ufs/ffs/ffs_snapshot.c
index 9aa0f791b5fc..c8d89698e8de 100644
--- a/sys/ufs/ffs/ffs_snapshot.c
+++ b/sys/ufs/ffs/ffs_snapshot.c
@@ -212,7 +212,7 @@ ffs_snapshot(mp, snapfile)
 	struct fs *copy_fs = NULL, *fs;
 	struct thread *td = curthread;
 	struct inode *ip, *xp;
-	struct buf *bp, *nbp, *ibp, *sbp = NULL;
+	struct buf *bp, *nbp, *ibp;
 	struct nameidata nd;
 	struct mount *wrtmp;
 	struct vattr vat;
@@ -460,21 +460,14 @@ restart:
 	 * Grab a copy of the superblock and its summary information.
 	 * We delay writing it until the suspension is released below.
 	 */
-	error = bread(vp, lblkno(fs, fs->fs_sblockloc), fs->fs_bsize,
-	    KERNCRED, &sbp);
-	if (error) {
-		brelse(sbp);
-		sbp = NULL;
-		goto out1;
-	}
-	loc = blkoff(fs, fs->fs_sblockloc);
-	copy_fs = (struct fs *)(sbp->b_data + loc);
+	copy_fs = malloc((u_long)fs->fs_bsize, M_UFSMNT, M_WAITOK);
 	bcopy(fs, copy_fs, fs->fs_sbsize);
 	if ((fs->fs_flags & (FS_UNCLEAN | FS_NEEDSFSCK)) == 0)
 		copy_fs->fs_clean = 1;
 	size = fs->fs_bsize < SBLOCKSIZE ? fs->fs_bsize : SBLOCKSIZE;
 	if (fs->fs_sbsize < size)
-		bzero(&sbp->b_data[loc + fs->fs_sbsize], size - fs->fs_sbsize);
+		bzero(&((char *)copy_fs)[fs->fs_sbsize],
+		    size - fs->fs_sbsize);
 	size = blkroundup(fs, fs->fs_cssize);
 	if (fs->fs_contigsumsize > 0)
 		size += fs->fs_ncg * sizeof(int32_t);
@@ -490,8 +483,8 @@ restart:
 		    len, KERNCRED, &bp)) != 0) {
 			brelse(bp);
 			free(copy_fs->fs_csp, M_UFSMNT);
-			bawrite(sbp);
-			sbp = NULL;
+			free(copy_fs, M_UFSMNT);
+			copy_fs = NULL;
 			goto out1;
 		}
 		bcopy(bp->b_data, space, (u_int)len);
@@ -606,8 +599,8 @@ loop:
 		vdrop(xvp);
 		if (error) {
 			free(copy_fs->fs_csp, M_UFSMNT);
-			bawrite(sbp);
-			sbp = NULL;
+			free(copy_fs, M_UFSMNT);
+			copy_fs = NULL;
 			MNT_VNODE_FOREACH_ABORT(mp, mvp);
 			goto out1;
 		}
@@ -621,8 +614,8 @@ loop:
 		error = softdep_journal_lookup(mp, &xvp);
 		if (error) {
 			free(copy_fs->fs_csp, M_UFSMNT);
-			bawrite(sbp);
-			sbp = NULL;
+			free(copy_fs, M_UFSMNT);
+			copy_fs = NULL;
 			goto out1;
 		}
 		xp = VTOI(xvp);
@@ -688,8 +681,8 @@ loop:
 	VI_UNLOCK(devvp);
 	ASSERT_VOP_LOCKED(vp, "ffs_snapshot vp");
 out1:
-	KASSERT((sn != NULL && sbp != NULL && error == 0) ||
-		(sn == NULL && sbp == NULL && error != 0),
+	KASSERT((sn != NULL && copy_fs != NULL && error == 0) ||
+		(sn == NULL && copy_fs == NULL && error != 0),
 		("email phk@ and mckusick@"));
 	/*
 	 * Resume operation on filesystem.
@@ -703,7 +696,7 @@ out1:
 		    vp->v_mount->mnt_stat.f_mntonname, (long)endtime.tv_sec,
 		    endtime.tv_nsec / 1000000, redo, fs->fs_ncg);
 	}
-	if (sbp == NULL)
+	if (copy_fs == NULL)
 		goto out;
 	/*
 	 * Copy allocation information from all the snapshots in
@@ -793,6 +786,15 @@ out1:
 		space = (char *)space + fs->fs_bsize;
 		bawrite(nbp);
 	}
+	error = bread(vp, lblkno(fs, fs->fs_sblockloc), fs->fs_bsize,
+	    KERNCRED, &nbp);
+	if (error) {
+		brelse(nbp);
+	} else {
+		loc = blkoff(fs, fs->fs_sblockloc);
+		bcopy((char *)copy_fs, &nbp->b_data[loc], fs->fs_bsize);
+		bawrite(nbp);
+	}
 	/*
 	 * As this is the newest list, it is the most inclusive, so
 	 * should replace the previous list.
@@ -822,7 +824,8 @@ out1:
 		vrele(vp);		/* Drop extra reference */
 done:
 	free(copy_fs->fs_csp, M_UFSMNT);
-	bawrite(sbp);
+	free(copy_fs, M_UFSMNT);
+	copy_fs = NULL;
 out:
 	NDFREE(&nd, NDF_ONLY_PNBUF);
 	if (saved_nice > 0) {