Resolve the devfs deadlock caused by LOR between devfs_mount->dm_lock and

vnode lock in devfs_allocv. Do this by temporary dropping dm_lock around
vnode locking.

For safe operation, add hold counters for both devfs_mount and devfs_dirent,
and DE_DOOMED flag for devfs_dirent. The facilities allow to continue after
dropping of the dm_lock, by making sure that referenced memory does not
disappear.

Reviewed by:	tegge
Tested by:	kris
Approved by:	kan (mentor)
PR:		kern/102335

1 files changed, 20 insertions, 4 deletions

diff --git a/sys/fs/devfs/devfs_vfsops.c b/sys/fs/devfs/devfs_vfsops.c
index 17db07a..8c1069a 100644
--- a/sys/fs/devfs/devfs_vfsops.c
+++ b/sys/fs/devfs/devfs_vfsops.c
@@ -77,6 +77,7 @@ devfs_mount(struct mount *mp, struct thread *td)
 	fmp = malloc(sizeof *fmp, M_DEVFS, M_WAITOK | M_ZERO);
 	fmp->dm_idx = alloc_unr(devfs_unr);
 	sx_init(&fmp->dm_lock, "devfsmount");
+	fmp->dm_holdcnt = 1;
 
 	mp->mnt_flag |= MNT_LOCAL;
 	mp->mnt_kern_flag |= MNTK_MPSAFE;
@@ -104,14 +105,25 @@ devfs_mount(struct mount *mp, struct thread *td)
 	return (0);
 }
 
+void
+devfs_unmount_final(struct devfs_mount *fmp)
+{
+	sx_destroy(&fmp->dm_lock);
+	free(fmp, M_DEVFS);
+}
+
 static int
 devfs_unmount(struct mount *mp, int mntflags, struct thread *td)
 {
 	int error;
 	int flags = 0;
 	struct devfs_mount *fmp;
+	int hold;
+	u_int idx;
 
 	fmp = VFSTODEVFS(mp);
+	KASSERT(fmp->dm_mount != NULL,
+		("devfs_unmount unmounted devfs_mount"));
 	/* There is 1 extra root vnode reference from devfs_mount(). */
 	error = vflush(mp, 1, flags, td);
 	if (error)
@@ -119,11 +131,14 @@ devfs_unmount(struct mount *mp, int mntflags, struct thread *td)
 	sx_xlock(&fmp->dm_lock);
 	devfs_cleanup(fmp);
 	devfs_rules_cleanup(fmp);
-	sx_xunlock(&fmp->dm_lock);
+	fmp->dm_mount = NULL;
+	hold = --fmp->dm_holdcnt;
 	mp->mnt_data = NULL;
-	sx_destroy(&fmp->dm_lock);
-	free_unr(devfs_unr, fmp->dm_idx);
-	free(fmp, M_DEVFS);
+	idx = fmp->dm_idx;
+	sx_xunlock(&fmp->dm_lock);
+	free_unr(devfs_unr, idx);
+	if (hold == 0)
+		devfs_unmount_final(fmp);
 	return 0;
 }
 
@@ -137,6 +152,7 @@ devfs_root(struct mount *mp, int flags, struct vnode **vpp, struct thread *td)
 	struct devfs_mount *dmp;
 
 	dmp = VFSTODEVFS(mp);
+	sx_xlock(&dmp->dm_lock);
 	error = devfs_allocv(dmp->dm_rootdir, mp, &vp, td);
 	if (error)
 		return (error);