From 128948d6ae94009c6ad13a0bd96e03e45a560477 Mon Sep 17 00:00:00 2001 From: Jamie Liu Date: Mon, 25 Nov 2019 15:20:25 -0800 Subject: Implement basic umounting for vfs2. This is required to test filesystems with a non-trivial implementation of FilesystemImpl.Release(). Propagation isn't handled yet, and umount isn't yet plumbed out to VirtualFilesystem.UmountAt(), but otherwise the implementation of umount is believed to be correct. - Move entering mountTable.seq writer critical sections to callers of mountTable.{insert,remove}Seqed. This is required since umount(2) must ensure that no new references are taken on the candidate mount after checking that it isn't busy, which is only possible by entering a vfs.mountTable.seq writer critical section before the check and remaining in it until after VFS.umountRecursiveLocked() is complete. (Linux does the same thing: fs/namespace.c:do_umount() => lock_mount_hash(), fs/pnode.c:propagate_mount_busy(), umount_tree(), unlock_mount_hash().) - It's not possible for dentry deletion to umount while only holding VFS.mountMu for reading, but it's also very unappealing to hold VFS.mountMu exclusively around e.g. gofer unlink RPCs. Introduce dentry.mu to avoid these problems. This means that VFS.mountMu is never acquired for reading, so change it to a sync.Mutex. PiperOrigin-RevId: 282444343 --- pkg/sentry/vfs/mount_unsafe.go | 60 ++++++++++++++++++++++++++---------------- 1 file changed, 37 insertions(+), 23 deletions(-) (limited to 'pkg/sentry/vfs/mount_unsafe.go') diff --git a/pkg/sentry/vfs/mount_unsafe.go b/pkg/sentry/vfs/mount_unsafe.go index c98b42f91..ab13fa461 100644 --- a/pkg/sentry/vfs/mount_unsafe.go +++ b/pkg/sentry/vfs/mount_unsafe.go @@ -38,16 +38,6 @@ type mountKey struct { point unsafe.Pointer // *Dentry } -// Invariant: mnt.key's fields are nil. parent and point are non-nil. -func (mnt *Mount) storeKey(parent *Mount, point *Dentry) { - atomic.StorePointer(&mnt.key.parent, unsafe.Pointer(parent)) - atomic.StorePointer(&mnt.key.point, unsafe.Pointer(point)) -} - -func (mnt *Mount) loadKey() (*Mount, *Dentry) { - return (*Mount)(atomic.LoadPointer(&mnt.key.parent)), (*Dentry)(atomic.LoadPointer(&mnt.key.point)) -} - func (mnt *Mount) parent() *Mount { return (*Mount)(atomic.LoadPointer(&mnt.key.parent)) } @@ -56,6 +46,19 @@ func (mnt *Mount) point() *Dentry { return (*Dentry)(atomic.LoadPointer(&mnt.key.point)) } +func (mnt *Mount) loadKey() VirtualDentry { + return VirtualDentry{ + mount: mnt.parent(), + dentry: mnt.point(), + } +} + +// Invariant: mnt.key.parent == nil. vd.Ok(). +func (mnt *Mount) storeKey(vd VirtualDentry) { + atomic.StorePointer(&mnt.key.parent, unsafe.Pointer(vd.mount)) + atomic.StorePointer(&mnt.key.point, unsafe.Pointer(vd.dentry)) +} + // mountTable maps (mount parent, mount point) pairs to mounts. It supports // efficient concurrent lookup, even in the presence of concurrent mutators // (provided mutation is sufficiently uncommon). @@ -201,9 +204,19 @@ loop: // Insert inserts the given mount into mt. // -// Preconditions: There are no concurrent mutators of mt. mt must not already -// contain a Mount with the same mount point and parent. +// Preconditions: mt must not already contain a Mount with the same mount point +// and parent. func (mt *mountTable) Insert(mount *Mount) { + mt.seq.BeginWrite() + mt.insertSeqed(mount) + mt.seq.EndWrite() +} + +// insertSeqed inserts the given mount into mt. +// +// Preconditions: mt.seq must be in a writer critical section. mt must not +// already contain a Mount with the same mount point and parent. +func (mt *mountTable) insertSeqed(mount *Mount) { hash := memhash(unsafe.Pointer(&mount.key), uintptr(mt.seed), mountKeyBytes) // We're under the maximum load factor if: @@ -215,10 +228,8 @@ func (mt *mountTable) Insert(mount *Mount) { tcap := uintptr(1) << order if ((tlen + 1) * mtMaxLoadDen) <= (uint64(mtMaxLoadNum) << order) { // Atomically insert the new element into the table. - mt.seq.BeginWrite() atomic.AddUint64(&mt.size, mtSizeLenOne) mtInsertLocked(mt.slots, tcap, unsafe.Pointer(mount), hash) - mt.seq.EndWrite() return } @@ -241,8 +252,6 @@ func (mt *mountTable) Insert(mount *Mount) { for { oldSlot := (*mountSlot)(oldCur) if oldSlot.value != nil { - // Don't need to lock mt.seq yet since newSlots isn't visible - // to readers. mtInsertLocked(newSlots, newCap, oldSlot.value, oldSlot.hash) } if oldCur == oldLast { @@ -252,11 +261,9 @@ func (mt *mountTable) Insert(mount *Mount) { } // Insert the new element into the new table. mtInsertLocked(newSlots, newCap, unsafe.Pointer(mount), hash) - // Atomically switch to the new table. - mt.seq.BeginWrite() + // Switch to the new table. atomic.AddUint64(&mt.size, mtSizeLenOne|mtSizeOrderOne) atomic.StorePointer(&mt.slots, newSlots) - mt.seq.EndWrite() } // Preconditions: There are no concurrent mutators of the table (slots, cap). @@ -294,9 +301,18 @@ func mtInsertLocked(slots unsafe.Pointer, cap uintptr, value unsafe.Pointer, has // Remove removes the given mount from mt. // -// Preconditions: There are no concurrent mutators of mt. mt must contain -// mount. +// Preconditions: mt must contain mount. func (mt *mountTable) Remove(mount *Mount) { + mt.seq.BeginWrite() + mt.removeSeqed(mount) + mt.seq.EndWrite() +} + +// removeSeqed removes the given mount from mt. +// +// Preconditions: mt.seq must be in a writer critical section. mt must contain +// mount. +func (mt *mountTable) removeSeqed(mount *Mount) { hash := memhash(unsafe.Pointer(&mount.key), uintptr(mt.seed), mountKeyBytes) tcap := uintptr(1) << (mt.size & mtSizeOrderMask) mask := tcap - 1 @@ -311,7 +327,6 @@ func (mt *mountTable) Remove(mount *Mount) { // backward until we either find an empty slot, or an element that // is already in its first-probed slot. (This is backward shift // deletion.) - mt.seq.BeginWrite() for { nextOff := (off + mountSlotBytes) & offmask nextSlot := (*mountSlot)(unsafe.Pointer(uintptr(slots) + nextOff)) @@ -330,7 +345,6 @@ func (mt *mountTable) Remove(mount *Mount) { } atomic.StorePointer(&slot.value, nil) atomic.AddUint64(&mt.size, mtSizeLenNegOne) - mt.seq.EndWrite() return } if checkInvariants && slotValue == nil { -- cgit v1.2.3