D31016.id92718.diff
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	diff --git a/sys/fs/nullfs/null.h b/sys/fs/nullfs/null.h
	--- a/sys/fs/nullfs/null.h
	+++ b/sys/fs/nullfs/null.h
	@@ -45,6 +45,8 @@
	struct mount *nullm_vfs;
	struct vnode nullm_lowerrootvp; / Ref to lower root vnode */
	uint64_t nullm_flags;
	+ struct mount_upper_node upper_node;
	+ struct mount_upper_node notify_node;
	};

	#ifdef _KERNEL
	diff --git a/sys/fs/nullfs/null_vfsops.c b/sys/fs/nullfs/null_vfsops.c
	--- a/sys/fs/nullfs/null_vfsops.c
	+++ b/sys/fs/nullfs/null_vfsops.c
	@@ -163,7 +163,8 @@
	* Save pointer to underlying FS and the reference to the
	* lower root vnode.
	*/
	- xmp->nullm_vfs = vfs_pin_from_vp(lowerrootvp);
	+ xmp->nullm_vfs = vfs_register_upper_from_vp(lowerrootvp, mp,
	+ &xmp->upper_node);
	if (xmp->nullm_vfs == NULL) {
	vput(lowerrootvp);
	free(xmp, M_NULLFSMNT);
	@@ -178,7 +179,7 @@
	*/
	error = null_nodeget(mp, lowerrootvp, &nullm_rootvp);
	if (error != 0) {
	- vfs_unpin(xmp->nullm_vfs);
	+ vfs_unregister_upper(xmp->nullm_vfs, &xmp->upper_node);
	vrele(lowerrootvp);
	free(xmp, M_NULLFSMNT);
	return (error);
	@@ -195,6 +196,11 @@
	(xmp->nullm_vfs->mnt_kern_flag & MNTK_NULL_NOCACHE) != 0)
	xmp->nullm_flags &= ~NULLM_CACHE;

	+ if ((xmp->nullm_flags & NULLM_CACHE) != 0) {
	+ vfs_register_for_notification(xmp->nullm_vfs, mp,
	+ &xmp->notify_node);
	+ }
	+
	MNT_ILOCK(mp);
	if ((xmp->nullm_flags & NULLM_CACHE) != 0) {
	mp->mnt_kern_flag \|= lowerrootvp->v_mount->mnt_kern_flag &
	@@ -206,13 +212,6 @@
	(MNTK_USES_BCACHE \| MNTK_NO_IOPF \| MNTK_UNMAPPED_BUFS);
	MNT_IUNLOCK(mp);
	vfs_getnewfsid(mp);
	- if ((xmp->nullm_flags & NULLM_CACHE) != 0) {
	- MNT_ILOCK(xmp->nullm_vfs);
	- TAILQ_INSERT_TAIL(&xmp->nullm_vfs->mnt_uppers, mp,
	- mnt_upper_link);
	- MNT_IUNLOCK(xmp->nullm_vfs);
	- }
	-
	vfs_mountedfrom(mp, target);
	vput(nullm_rootvp);

	@@ -230,7 +229,6 @@
	int mntflags;
	{
	struct null_mount *mntdata;
	- struct mount *ump;
	int error, flags;

	NULLFSDEBUG("nullfs_unmount: mp = %p\n", (void *)mp);
	@@ -259,17 +257,11 @@
	* Finally, throw away the null_mount structure
	*/
	mntdata = mp->mnt_data;
	- ump = mntdata->nullm_vfs;
	if ((mntdata->nullm_flags & NULLM_CACHE) != 0) {
	- MNT_ILOCK(ump);
	- while ((ump->mnt_kern_flag & MNTK_VGONE_UPPER) != 0) {
	- ump->mnt_kern_flag \|= MNTK_VGONE_WAITER;
	- msleep(&ump->mnt_uppers, &ump->mnt_mtx, 0, "vgnupw", 0);
	- }
	- TAILQ_REMOVE(&ump->mnt_uppers, mp, mnt_upper_link);
	- MNT_IUNLOCK(ump);
	+ vfs_unregister_for_notification(mntdata->nullm_vfs,
	+ &mntdata->notify_node);
	}
	- vfs_unpin(ump);
	+ vfs_unregister_upper(mntdata->nullm_vfs, &mntdata->upper_node);
	vrele(mntdata->nullm_lowerrootvp);
	mp->mnt_data = NULL;
	free(mntdata, M_NULLFSMNT);
	diff --git a/sys/fs/unionfs/union.h b/sys/fs/unionfs/union.h
	--- a/sys/fs/unionfs/union.h
	+++ b/sys/fs/unionfs/union.h
	@@ -57,6 +57,8 @@
	struct vnode um_lowervp; / VREFed once */
	struct vnode um_uppervp; / VREFed once */
	struct vnode um_rootvp; / ROOT vnode */
	+ struct mount_upper_node um_lower_link; /* node in lower FS list of uppers */
	+ struct mount_upper_node um_upper_link; /* node in upper FS list of uppers */
	unionfs_copymode um_copymode;
	unionfs_whitemode um_whitemode;
	uid_t um_uid;
	diff --git a/sys/fs/unionfs/union_vfsops.c b/sys/fs/unionfs/union_vfsops.c
	--- a/sys/fs/unionfs/union_vfsops.c
	+++ b/sys/fs/unionfs/union_vfsops.c
	@@ -292,14 +292,16 @@
	return (error);
	}

	- lowermp = vfs_pin_from_vp(ump->um_lowervp);
	- uppermp = vfs_pin_from_vp(ump->um_uppervp);
	+ lowermp = vfs_register_upper_from_vp(ump->um_lowervp, mp,
	+ &ump->um_lower_link);
	+ uppermp = vfs_register_upper_from_vp(ump->um_uppervp, mp,
	+ &ump->um_upper_link);

	if (lowermp == NULL \|\| uppermp == NULL) {
	if (lowermp != NULL)
	- vfs_unpin(lowermp);
	+ vfs_unregister_upper(lowermp, &ump->um_lower_link);
	if (uppermp != NULL)
	- vfs_unpin(uppermp);
	+ vfs_unregister_upper(uppermp, &ump->um_upper_link);
	free(ump, M_UNIONFSMNT);
	mp->mnt_data = NULL;
	return (ENOENT);
	@@ -357,8 +359,8 @@
	if (error)
	return (error);

	- vfs_unpin(ump->um_lowervp->v_mount);
	- vfs_unpin(ump->um_uppervp->v_mount);
	+ vfs_unregister_upper(ump->um_lowervp->v_mount, &ump->um_lower_link);
	+ vfs_unregister_upper(ump->um_uppervp->v_mount, &ump->um_upper_link);
	free(ump, M_UNIONFSMNT);
	mp->mnt_data = NULL;

	diff --git a/sys/kern/vfs_mount.c b/sys/kern/vfs_mount.c
	--- a/sys/kern/vfs_mount.c
	+++ b/sys/kern/vfs_mount.c
	@@ -65,6 +65,7 @@
	#include <sys/sysctl.h>
	#include <sys/sysent.h>
	#include <sys/systm.h>
	+#include <sys/taskqueue.h>
	#include <sys/vnode.h>
	#include <vm/uma.h>

	@@ -89,6 +90,11 @@
	SYSCTL_BOOL(_vfs, OID_AUTO, default_autoro, CTLFLAG_RW, &default_autoro, 0,
	"Retry failed r/w mount as r/o if no explicit ro/rw option is specified");

	+static bool recursive_forced_unmount = false;
	+SYSCTL_BOOL(_vfs, OID_AUTO, recursive_forced_unmount, CTLFLAG_RW,
	+ &recursive_forced_unmount, 0, "Recursively unmount stacked upper mounts"
	+ " when a file system is forcibly unmounted");
	+
	MALLOC_DEFINE(M_MOUNT, "mount", "vfs mount structure");
	MALLOC_DEFINE(M_STATFS, "statfs", "statfs structure");
	static uma_zone_t mount_zone;
	@@ -103,6 +109,16 @@
	EVENTHANDLER_LIST_DEFINE(vfs_mounted);
	EVENTHANDLER_LIST_DEFINE(vfs_unmounted);

	+static void vfs_deferred_unmount(void *arg, int pending);
	+static struct task deferred_unmount_task =
	+ TASK_INITIALIZER(0, vfs_deferred_unmount, NULL);;
	+static struct mtx deferred_unmount_lock;
	+MTX_SYSINIT(deferred_unmount, &deferred_unmount_lock, "deferred_unmount",
	+ MTX_DEF);
	+static STAILQ_HEAD(, mount) deferred_unmount_list =
	+ STAILQ_HEAD_INITIALIZER(deferred_unmount_list);
	+TASKQUEUE_DEFINE_THREAD(deferred_unmount);
	+
	static void mount_devctl_event(const char type, struct mount mp, bool donew);

	/*
	@@ -505,8 +521,21 @@
	MNT_IUNLOCK(mp);
	}

	+/*
	+ * Register ump as an upper mount of the mount associated with
	+ * vnode vp. This registration will be tracked through
	+ * mount_upper_node upper, which should be allocated by the
	+ * caller and stored in per-mount data associated with mp.
	+ *
	+ * If successful, this function will return the mount associated
	+ * with vp, and will ensure that it cannot be unmounted until
	+ * ump has been unregistered as one of its upper mounts.
	+ *
	+ * Upon failure this function will return NULL.
	+ */
	struct mount *
	-vfs_pin_from_vp(struct vnode *vp)
	+vfs_register_upper_from_vp(struct vnode vp, struct mount ump,
	+ struct mount_upper_node *upper)
	{
	struct mount *mp;

	@@ -514,26 +543,81 @@
	if (mp == NULL)
	return (NULL);
	MNT_ILOCK(mp);
	- if (mp != vp->v_mount \|\| (mp->mnt_kern_flag & MNTK_UNMOUNT) != 0) {
	+ if (mp != vp->v_mount \|\|
	+ ((mp->mnt_kern_flag & (MNTK_UNMOUNT \| MNTK_RECURSE)) != 0)) {
	MNT_IUNLOCK(mp);
	return (NULL);
	}
	+ KASSERT(ump != mp, ("upper and lower mounts are identical"));
	+ upper->mp = ump;
	MNT_REF(mp);
	- KASSERT(mp->mnt_pinned_count < INT_MAX,
	- ("mount pinned count overflow"));
	- ++mp->mnt_pinned_count;
	+ TAILQ_INSERT_TAIL(&mp->mnt_uppers, upper, mnt_upper_link);
	MNT_IUNLOCK(mp);
	return (mp);
	}

	+/*
	+ * Register upper mount ump to receive vnode unlink/reclaim
	+ * notifications from lower mount mp. This registration will
	+ * be tracked through mount_upper_node upper, which should be
	+ * allocated by the caller and stored in per-mount data
	+ * associated with mp.
	+ *
	+ * ump must already be registered as an upper mount of mp
	+ * through a call to vfs_register_upper_from_vp().
	+ */
	void
	-vfs_unpin(struct mount *mp)
	+vfs_register_for_notification(struct mount mp, struct mount ump,
	+ struct mount_upper_node *upper)
	+{
	+ upper->mp = ump;
	+ MNT_ILOCK(mp);
	+ TAILQ_INSERT_TAIL(&mp->mnt_notify, upper, mnt_upper_link);
	+ MNT_IUNLOCK(mp);
	+}
	+
	+static void
	+vfs_drain_upper_locked(struct mount *mp)
	+{
	+ mtx_assert(MNT_MTX(mp), MA_OWNED);
	+ while (mp->mnt_upper_pending != 0) {
	+ mp->mnt_kern_flag \|= MNTK_UPPER_WAITER;
	+ msleep(&mp->mnt_uppers, MNT_MTX(mp), 0, "mntupw", 0);
	+ }
	+}
	+
	+/*
	+ * Undo a previous call to vfs_register_for_notification().
	+ * The mount represented by upper must be currently registered
	+ * as an upper mount for mp.
	+ */
	+void
	+vfs_unregister_for_notification(struct mount *mp,
	+ struct mount_upper_node *upper)
	+{
	+ MNT_ILOCK(mp);
	+ vfs_drain_upper_locked(mp);
	+ TAILQ_REMOVE(&mp->mnt_notify, upper, mnt_upper_link);
	+ MNT_IUNLOCK(mp);
	+}
	+
	+/*
	+ * Undo a previous call to vfs_register_upper_from_vp().
	+ * This must be done before mp can be unmounted.
	+ */
	+void
	+vfs_unregister_upper(struct mount mp, struct mount_upper_node upper)
	{
	MNT_ILOCK(mp);
	- KASSERT(mp->mnt_pinned_count > 0, ("mount pinned count underflow"));
	KASSERT((mp->mnt_kern_flag & MNTK_UNMOUNT) == 0,
	- ("mount pinned with pending unmount"));
	- --mp->mnt_pinned_count;
	+ ("registered upper with pending unmount"));
	+ vfs_drain_upper_locked(mp);
	+ TAILQ_REMOVE(&mp->mnt_uppers, upper, mnt_upper_link);
	+ if ((mp->mnt_kern_flag & MNTK_TASKQUEUE_WAITER) != 0 &&
	+ TAILQ_EMPTY(&mp->mnt_uppers)) {
	+ mp->mnt_kern_flag &= ~MNTK_TASKQUEUE_WAITER;
	+ wakeup(&mp->mnt_taskqueue_link);
	+ }
	MNT_REL(mp);
	MNT_IUNLOCK(mp);
	}
	@@ -600,8 +684,10 @@
	mac_mount_create(cred, mp);
	#endif
	arc4rand(&mp->mnt_hashseed, sizeof mp->mnt_hashseed, 0);
	+ mp->mnt_upper_pending = 0;
	TAILQ_INIT(&mp->mnt_uppers);
	- mp->mnt_pinned_count = 0;
	+ TAILQ_INIT(&mp->mnt_notify);
	+ mp->mnt_taskqueue_flags = 0;
	return (mp);
	}

	@@ -640,9 +726,9 @@
	vn_printf(vp, "dangling vnode ");
	panic("unmount: dangling vnode");
	}
	- KASSERT(mp->mnt_pinned_count == 0,
	- ("mnt_pinned_count = %d", mp->mnt_pinned_count));
	+ KASSERT(mp->mnt_upper_pending == 0, ("mnt_upper_pending"));
	KASSERT(TAILQ_EMPTY(&mp->mnt_uppers), ("mnt_uppers"));
	+ KASSERT(TAILQ_EMPTY(&mp->mnt_notify), ("mnt_notify"));
	if (mp->mnt_nvnodelistsize != 0)
	panic("vfs_mount_destroy: nonzero nvnodelistsize");
	if (mp->mnt_lazyvnodelistsize != 0)
	@@ -1799,17 +1885,166 @@
	return (sum);
	}

	+static bool
	+deferred_unmount_enqueue(struct mount *mp, uint64_t flags, bool requeue)
	+{
	+ bool enqueued;
	+
	+ enqueued = false;
	+ mtx_lock(&deferred_unmount_lock);
	+ if ((mp->mnt_taskqueue_flags & MNT_DEFERRED) == 0 \|\| requeue) {
	+ mp->mnt_taskqueue_flags = flags \| MNT_DEFERRED;
	+ STAILQ_INSERT_TAIL(&deferred_unmount_list, mp,
	+ mnt_taskqueue_link);
	+ enqueued = true;
	+ }
	+ mtx_unlock(&deferred_unmount_lock);
	+
	+ if (enqueued) {
	+ taskqueue_enqueue(taskqueue_deferred_unmount,
	+ &deferred_unmount_task);
	+ }
	+
	+ return (enqueued);
	+}
	+
	+/*
	+ * Taskqueue handler for processing async/recursive unmounts
	+ */
	+static void
	+vfs_deferred_unmount(void *argi __unused, int pending __unused)
	+{
	+ STAILQ_HEAD(, mount) local_unmounts;
	+ uint64_t flags;
	+ struct mount mp, tmp;
	+ bool unmounted;
	+
	+ STAILQ_INIT(&local_unmounts);
	+ mtx_lock(&deferred_unmount_lock);
	+ STAILQ_CONCAT(&local_unmounts, &deferred_unmount_list);
	+ mtx_unlock(&deferred_unmount_lock);
	+
	+ STAILQ_FOREACH_SAFE(mp, &local_unmounts, mnt_taskqueue_link, tmp) {
	+ flags = mp->mnt_taskqueue_flags;
	+ KASSERT((flags & MNT_DEFERRED) != 0,
	+ ("taskqueue unmount without MNT_DEFERRED"));
	+ if (dounmount(mp, flags, curthread) != 0) {
	+ MNT_ILOCK(mp);
	+ unmounted = ((mp->mnt_kern_flag & MNTK_REFEXPIRE) != 0);
	+ MNT_IUNLOCK(mp);
	+ if (!unmounted)
	+ deferred_unmount_enqueue(mp, flags, true);
	+ else
	+ vfs_rel(mp);
	+ }
	+ }
	+}
	+
	/*
	* Do the actual filesystem unmount.
	*/
	int
	-dounmount(struct mount mp, int flags, struct thread td)
	+dounmount(struct mount mp, uint64_t flags, struct thread td)
	{
	+ struct mount_upper_node *upper;
	struct vnode coveredvp, rootvp;
	int error;
	uint64_t async_flag;
	int mnt_gen_r;

	+ KASSERT((flags & MNT_DEFERRED) == 0 \|\|
	+ (flags & (MNT_RECURSE \| MNT_FORCE)) == (MNT_RECURSE \| MNT_FORCE),
	+ ("MNT_DEFERRED requires MNT_RECURSE \| MNT_FORCE"));
	+
	+ /*
	+ * If the caller has explicitly requested the unmount to be handled by
	+ * the taskqueue and we're not already in taskqueue context, queue
	+ * up the unmount request and exit. This is done prior to any
	+ * credential checks; MNT_DEFERRED should be used only for kernel-
	+ * initiated unmounts and will therefore be processed with the
	+ * (kernel) credentials of the taskqueue thread. Still, callers
	+ * should be sure this is the behavior they want.
	+ */
	+ if ((flags & MNT_DEFERRED) != 0 &&
	+ taskqueue_member(taskqueue_deferred_unmount, curthread) == 0) {
	+ if (!deferred_unmount_enqueue(mp, flags, false))
	+ vfs_rel(mp);
	+ return (EINPROGRESS);
	+ }
	+
	+ /*
	+ * Only privileged root, or (if MNT_USER is set) the user that did the
	+ * original mount is permitted to unmount this filesystem.
	+ * This check should be made prior to queueing up any recursive
	+ * unmounts of upper filesystems. Those unmounts will be executed
	+ * with kernel thread credentials and are expected to succeed, so
	+ * we must at least ensure the originating context has sufficient
	+ * privilege to unmount the base filesystem before proceeding with
	+ * the uppers.
	+ */
	+ error = vfs_suser(mp, td);
	+ if (error != 0) {
	+ KASSERT((flags & MNT_DEFERRED) == 0,
	+ ("taskqueue unmount with insufficient privilege"));
	+ vfs_rel(mp);
	+ return (error);
	+ }
	+
	+ if (recursive_forced_unmount && ((flags & MNT_FORCE) != 0))
	+ flags \|= MNT_RECURSE;
	+
	+ if ((flags & MNT_RECURSE) != 0) {
	+ KASSERT((flags & MNT_FORCE) != 0,
	+ ("MNT_RECURSE requires MNT_FORCE"));
	+
	+ MNT_ILOCK(mp);
	+ /*
	+ * Set MNTK_RECURSE to prevent new upper mounts from being
	+ * added, and note that an operation on the uppers list is in
	+ * progress. This will ensure that unregistration from the
	+ * uppers list, and therefore any pending unmount of the upper
	+ * FS, can't complete until after we finish walking the list.
	+ */
	+ mp->mnt_kern_flag \|= MNTK_RECURSE;
	+ mp->mnt_upper_pending++;
	+ TAILQ_FOREACH(upper, &mp->mnt_uppers, mnt_upper_link) {
	+ MNT_IUNLOCK(mp);
	+ vfs_ref(upper->mp);
	+ if (!deferred_unmount_enqueue(upper->mp, flags, false))
	+ vfs_rel(upper->mp);
	+ MNT_ILOCK(mp);
	+ }
	+ mp->mnt_upper_pending--;
	+ if ((mp->mnt_kern_flag & MNTK_UPPER_WAITER) != 0 &&
	+ mp->mnt_upper_pending == 0) {
	+ mp->mnt_kern_flag &= ~MNTK_UPPER_WAITER;
	+ wakeup(&mp->mnt_uppers);
	+ }
	+ /*
	+ * If we're not on the taskqueue, wait until the uppers list
	+ * is drained before proceeding with unmount. Otherwise, if
	+ * we are on the taskqueue and there are still pending uppers,
	+ * just re-enqueue on the end of the taskqueue.
	+ */
	+ if ((flags & MNT_DEFERRED) == 0) {
	+ while (!TAILQ_EMPTY(&mp->mnt_uppers)) {
	+ mp->mnt_kern_flag \|= MNTK_TASKQUEUE_WAITER;
	+ msleep(&mp->mnt_taskqueue_link, MNT_MTX(mp), 0,
	+ "umntqw", 0);
	+ }
	+ } else if (!TAILQ_EMPTY(&mp->mnt_uppers)) {
	+ MNT_IUNLOCK(mp);
	+ deferred_unmount_enqueue(mp, flags, true);
	+ return (0);
	+ }
	+ MNT_IUNLOCK(mp);
	+ KASSERT(TAILQ_EMPTY(&mp->mnt_uppers), ("mnt_uppers not empty"));
	+ }
	+
	+ /* Allow the taskqueue to safely re-enqueue on failure */
	+ if ((flags & MNT_DEFERRED) != 0)
	+ vfs_ref(mp);
	+
	if ((coveredvp = mp->mnt_vnodecovered) != NULL) {
	mnt_gen_r = mp->mnt_gen;
	VI_LOCK(coveredvp);
	@@ -1828,27 +2063,13 @@
	}
	}

	- /*
	- * Only privileged root, or (if MNT_USER is set) the user that did the
	- * original mount is permitted to unmount this filesystem.
	- */
	- error = vfs_suser(mp, td);
	- if (error != 0) {
	- if (coveredvp != NULL) {
	- VOP_UNLOCK(coveredvp);
	- vdrop(coveredvp);
	- }
	- vfs_rel(mp);
	- return (error);
	- }
	-
	vfs_op_enter(mp);

	vn_start_write(NULL, &mp, V_WAIT \| V_MNTREF);
	MNT_ILOCK(mp);
	if ((mp->mnt_kern_flag & MNTK_UNMOUNT) != 0 \|\|
	(mp->mnt_flag & MNT_UPDATE) != 0 \|\|
	- mp->mnt_pinned_count != 0) {
	+ !TAILQ_EMPTY(&mp->mnt_uppers)) {
	dounmount_cleanup(mp, coveredvp, 0);
	return (EBUSY);
	}
	@@ -1952,6 +2173,7 @@
	}
	return (error);
	}
	+
	mtx_lock(&mountlist_mtx);
	TAILQ_REMOVE(&mountlist, mp, mnt_list);
	mtx_unlock(&mountlist_mtx);
	@@ -1977,6 +2199,8 @@
	}
	if (mp == rootdevmp)
	rootdevmp = NULL;
	+ if ((flags & MNT_DEFERRED) != 0)
	+ vfs_rel(mp);
	vfs_mount_destroy(mp);
	return (0);
	}
	diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
	--- a/sys/kern/vfs_subr.c
	+++ b/sys/kern/vfs_subr.c
	@@ -831,9 +831,9 @@
	* valid.
	*/
	while (mp->mnt_kern_flag & MNTK_UNMOUNT) {
	- KASSERT(mp->mnt_pinned_count == 0,
	- ("%s: non-zero pinned count %d with pending unmount",
	- __func__, mp->mnt_pinned_count));
	+ KASSERT(TAILQ_EMPTY(&mp->mnt_uppers),
	+ ("%s: non-empty upper mount list with pending unmount",
	+ __func__));
	if (flags & MBF_NOWAIT \|\| mp->mnt_kern_flag & MNTK_REFEXPIRE) {
	MNT_REL(mp);
	MNT_IUNLOCK(mp);
	@@ -3897,6 +3897,11 @@
	{
	}

	+struct notify_mount {
	+ struct mount mp;
	+ struct mount_upper_node upper;
	+};
	+
	/*
	* Notify upper mounts about reclaimed or unlinked vnode.
	*/
	@@ -3907,45 +3912,52 @@
	.vfs_reclaim_lowervp = notify_lowervp_vfs_dummy,
	.vfs_unlink_lowervp = notify_lowervp_vfs_dummy,
	};
	- struct mount mp, ump, *mmp;
	+ struct mount *mp;
	+ struct mount_upper_node *ump;
	+ struct notify_mount *mmp;

	mp = vp->v_mount;
	if (mp == NULL)
	return;
	- if (TAILQ_EMPTY(&mp->mnt_uppers))
	+ if (TAILQ_EMPTY(&mp->mnt_notify))
	return;

	- mmp = malloc(sizeof(struct mount), M_TEMP, M_WAITOK \| M_ZERO);
	- mmp->mnt_op = &vgonel_vfsops;
	- mmp->mnt_kern_flag \|= MNTK_MARKER;
	+ mmp = malloc(sizeof(*mmp), M_TEMP, M_WAITOK \| M_ZERO);
	+ mmp->mp.mnt_op = &vgonel_vfsops;
	+ mmp->mp.mnt_kern_flag \|= MNTK_MARKER;
	+ mmp->upper.mp = &mmp->mp;
	MNT_ILOCK(mp);
	- mp->mnt_kern_flag \|= MNTK_VGONE_UPPER;
	- for (ump = TAILQ_FIRST(&mp->mnt_uppers); ump != NULL;) {
	- if ((ump->mnt_kern_flag & MNTK_MARKER) != 0) {
	+ mp->mnt_upper_pending++;
	+ KASSERT(mp->mnt_upper_pending > 0,
	+ ("%s: mnt_upper_pending %d", __func__, mp->mnt_upper_pending));
	+ for (ump = TAILQ_FIRST(&mp->mnt_notify); ump != NULL;) {
	+ if ((ump->mp->mnt_kern_flag & MNTK_MARKER) != 0) {
	ump = TAILQ_NEXT(ump, mnt_upper_link);
	continue;
	}
	- TAILQ_INSERT_AFTER(&mp->mnt_uppers, ump, mmp, mnt_upper_link);
	+ TAILQ_INSERT_AFTER(&mp->mnt_notify, ump, &mmp->upper,
	+ mnt_upper_link);
	MNT_IUNLOCK(mp);
	switch (event) {
	case VFS_NOTIFY_UPPER_RECLAIM:
	- VFS_RECLAIM_LOWERVP(ump, vp);
	+ VFS_RECLAIM_LOWERVP(ump->mp, vp);
	break;
	case VFS_NOTIFY_UPPER_UNLINK:
	- VFS_UNLINK_LOWERVP(ump, vp);
	+ VFS_UNLINK_LOWERVP(ump->mp, vp);
	break;
	default:
	KASSERT(0, ("invalid event %d", event));
	break;
	}
	MNT_ILOCK(mp);
	- ump = TAILQ_NEXT(mmp, mnt_upper_link);
	- TAILQ_REMOVE(&mp->mnt_uppers, mmp, mnt_upper_link);
	+ ump = TAILQ_NEXT(&mmp->upper, mnt_upper_link);
	+ TAILQ_REMOVE(&mp->mnt_notify, &mmp->upper, mnt_upper_link);
	}
	free(mmp, M_TEMP);
	- mp->mnt_kern_flag &= ~MNTK_VGONE_UPPER;
	- if ((mp->mnt_kern_flag & MNTK_VGONE_WAITER) != 0) {
	- mp->mnt_kern_flag &= ~MNTK_VGONE_WAITER;
	+ mp->mnt_upper_pending--;
	+ if ((mp->mnt_kern_flag & MNTK_UPPER_WAITER) != 0 &&
	+ mp->mnt_upper_pending == 0) {
	+ mp->mnt_kern_flag &= ~MNTK_UPPER_WAITER;
	wakeup(&mp->mnt_uppers);
	}
	MNT_IUNLOCK(mp);
	@@ -4376,12 +4388,13 @@
	MNT_KERN_FLAG(MNTK_EXTENDED_SHARED);
	MNT_KERN_FLAG(MNTK_SHARED_WRITES);
	MNT_KERN_FLAG(MNTK_NO_IOPF);
	- MNT_KERN_FLAG(MNTK_VGONE_UPPER);
	- MNT_KERN_FLAG(MNTK_VGONE_WAITER);
	+ MNT_KERN_FLAG(MNTK_RECURSE);
	+ MNT_KERN_FLAG(MNTK_UPPER_WAITER);
	MNT_KERN_FLAG(MNTK_LOOKUP_EXCL_DOTDOT);
	MNT_KERN_FLAG(MNTK_MARKER);
	MNT_KERN_FLAG(MNTK_USES_BCACHE);
	MNT_KERN_FLAG(MNTK_FPLOOKUP);
	+ MNT_KERN_FLAG(MNTK_TASKQUEUE_WAITER);
	MNT_KERN_FLAG(MNTK_NOASYNC);
	MNT_KERN_FLAG(MNTK_UNMOUNT);
	MNT_KERN_FLAG(MNTK_MWAIT);
	diff --git a/sys/sys/mount.h b/sys/sys/mount.h
	--- a/sys/sys/mount.h
	+++ b/sys/sys/mount.h
	@@ -190,6 +190,19 @@
	_Static_assert(sizeof(struct mount_pcpu) == 16,
	"the struct is allocated from pcpu 16 zone");

	+/*
	+ * Structure for tracking a stacked filesystem mounted above another
	+ * filesystem. This is expected to be stored in the upper FS' per-mount data.
	+ *
	+ * Lock reference:
	+ * i - lower mount interlock
	+ * c - constant from node initialization
	+ */
	+struct mount_upper_node {
	+ struct mount mp; / (c) mount object for upper FS */
	+ TAILQ_ENTRY(mount_upper_node) mnt_upper_link; /* (i) position in uppers list */
	+};
	+
	/*
	* Structure per mounted filesystem. Each mounted filesystem has an
	* array of operations and an instance record. The filesystems are
	@@ -199,8 +212,8 @@
	* l - mnt_listmtx
	* m - mountlist_mtx
	* i - interlock
	- * i* - interlock of uppers' list head
	* v - vnode freelist mutex
	+ * d - deferred unmount list mutex
	*
	* Unmarked fields are considered stable as long as a ref is held.
	*
	@@ -242,10 +255,12 @@
	struct mtx mnt_listmtx;
	struct vnodelst mnt_lazyvnodelist; /* (l) list of lazy vnodes */
	int mnt_lazyvnodelistsize; /* (l) # of lazy vnodes */
	- int mnt_pinned_count; /* (i) unmount prevented */
	+ int mnt_upper_pending; /* (i) # of pending ops on mnt_uppers */
	struct lock mnt_explock; /* vfs_export walkers lock */
	- TAILQ_ENTRY(mount) mnt_upper_link; /* (i) we in the all uppers /
	- TAILQ_HEAD(, mount) mnt_uppers; /* (i) upper mounts over us */
	+ TAILQ_HEAD(, mount_upper_node) mnt_uppers; /* (i) upper mounts over us */
	+ TAILQ_HEAD(, mount_upper_node) mnt_notify; /* (i) upper mounts for notification */
	+ STAILQ_ENTRY(mount) mnt_taskqueue_link; /* (d) our place in deferred unmount list */
	+ uint64_t mnt_taskqueue_flags; /* (d) unmount flags passed from taskqueue */
	};
	#endif /* _WANT_MOUNT \|\| _KERNEL */

	@@ -438,9 +453,13 @@
	#define MNT_BYFSID 0x0000000008000000ULL /* specify filesystem by ID. */
	#define MNT_NOCOVER 0x0000001000000000ULL /* Do not cover a mount point */
	#define MNT_EMPTYDIR 0x0000002000000000ULL /* Only mount on empty dir */
	-#define MNT_CMDFLAGS (MNT_UPDATE \| MNT_DELEXPORT \| MNT_RELOAD \| \
	+#define MNT_RECURSE 0x0000100000000000ULL /* recursively unmount uppers */
	+#define MNT_DEFERRED 0x0000200000000000ULL /* unmount in async context */
	+#define MNT_CMDFLAGS (MNT_UPDATE \| MNT_DELEXPORT \| MNT_RELOAD \| \
	MNT_FORCE \| MNT_SNAPSHOT \| MNT_NONBUSY \| \
	- MNT_BYFSID \| MNT_NOCOVER \| MNT_EMPTYDIR)
	+ MNT_BYFSID \| MNT_NOCOVER \| MNT_EMPTYDIR \| \
	+ MNT_RECURSE \| MNT_DEFERRED)
	+
	/*
	* Internal filesystem control flags stored in mnt_kern_flag.
	*
	@@ -466,8 +485,8 @@
	#define MNTK_NO_IOPF 0x00000100 /* Disallow page faults during reads
	and writes. Filesystem shall properly
	handle i/o state on EFAULT. */
	-#define MNTK_VGONE_UPPER 0x00000200
	-#define MNTK_VGONE_WAITER 0x00000400
	+#define MNTK_RECURSE 0x00000200 /* pending recursive unmount */
	+#define MNTK_UPPER_WAITER 0x00000400 /* waiting to drain MNTK_UPPER_PENDING */
	#define MNTK_LOOKUP_EXCL_DOTDOT 0x00000800
	#define MNTK_MARKER 0x00001000
	#define MNTK_UNMAPPED_BUFS 0x00002000
	@@ -477,8 +496,9 @@
	#define MNTK_UNIONFS 0x00020000 /* A hack for F_ISUNIONSTACK */
	#define MNTK_FPLOOKUP 0x00040000 /* fast path lookup is supported */
	#define MNTK_SUSPEND_ALL 0x00080000 /* Suspended by all-fs suspension */
	-#define MNTK_NOASYNC 0x00800000 /* disable async */
	-#define MNTK_UNMOUNT 0x01000000 /* unmount in progress */
	+#define MNTK_TASKQUEUE_WAITER 0x00100000 /* Waiting on unmount taskqueue */
	+#define MNTK_NOASYNC 0x00800000 /* disable async */
	+#define MNTK_UNMOUNT 0x01000000 /* unmount in progress */
	#define MNTK_MWAIT 0x02000000 /* waiting for unmount to finish */
	#define MNTK_SUSPEND 0x08000000 /* request write suspension */
	#define MNTK_SUSPEND2 0x04000000 /* block secondary writes */
	@@ -952,7 +972,7 @@
	* exported vnode operations
	*/

	-int dounmount(struct mount , int, struct thread );
	+int dounmount(struct mount , uint64_t, struct thread );

	int kernel_mount(struct mntarg *ma, uint64_t flags);
	int kernel_vmount(int flags, ...);
	@@ -1012,8 +1032,13 @@
	int vfs_suser(struct mount , struct thread );
	void vfs_unbusy(struct mount *);
	void vfs_unmountall(void);
	-struct mount vfs_pin_from_vp(struct vnode );
	-void vfs_unpin(struct mount *);
	+struct mount vfs_register_upper_from_vp(struct vnode ,
	+ struct mount ump, struct mount_upper_node );
	+void vfs_register_for_notification(struct mount , struct mount ,
	+ struct mount_upper_node *);
	+void vfs_unregister_for_notification(struct mount *,
	+ struct mount_upper_node *);
	+void vfs_unregister_upper(struct mount , struct mount_upper_node );
	extern TAILQ_HEAD(mntlist, mount) mountlist; /* mounted filesystem list */
	extern struct mtx_padalign mountlist_mtx;
	extern struct nfs_public nfs_pub;
	diff --git a/sys/ufs/ffs/ffs_vfsops.c b/sys/ufs/ffs/ffs_vfsops.c
	--- a/sys/ufs/ffs/ffs_vfsops.c
	+++ b/sys/ufs/ffs/ffs_vfsops.c
	@@ -297,7 +297,7 @@
	*/
	mp = vfs_getvfs(&etp->fsid);
	if (mp != NULL)
	- dounmount(mp, MNT_FORCE, curthread);
	+ dounmount(mp, MNT_FORCE \| MNT_RECURSE, curthread);
	free(etp, M_UFSMNT);
	}

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