path: root/pkg/sentry/kernel/futex/futex.go

// Copyright 2018 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package futex provides an implementation of the futex interface as found in
// the Linux kernel. It allows one to easily transform Wait() calls into waits
// on a channel, which is useful in a Go-based kernel, for example.
package futex

import (
	"sync"

	"gvisor.googlesource.com/gvisor/pkg/sentry/memmap"
	"gvisor.googlesource.com/gvisor/pkg/syserror"
)

// KeyKind indicates the type of a Key.
type KeyKind int

const (
	// KindPrivate indicates a private futex (a futex syscall with the
	// FUTEX_PRIVATE_FLAG set).
	KindPrivate KeyKind = iota

	// KindSharedPrivate indicates a shared futex on a private memory mapping.
	// Although KindPrivate and KindSharedPrivate futexes both use memory
	// addresses to identify futexes, they do not interoperate (in Linux, the
	// two are distinguished by the FUT_OFF_MMSHARED flag, which is used in key
	// comparison).
	KindSharedPrivate

	// KindSharedMappable indicates a shared futex on a memory mapping other
	// than a private anonymous memory mapping.
	KindSharedMappable
)

// Key represents something that a futex waiter may wait on.
type Key struct {
	// Kind is the type of the Key.
	Kind KeyKind

	// Mappable is the memory-mapped object that is represented by the Key.
	// Mappable is always nil if Kind is not KindSharedMappable, and may be nil
	// even if it is.
	Mappable memmap.Mappable

	// MappingIdentity is the MappingIdentity associated with Mappable.
	// MappingIdentity is always nil is Mappable is nil, and may be nil even if
	// it isn't.
	MappingIdentity memmap.MappingIdentity

	// If Kind is KindPrivate or KindSharedPrivate, Offset is the represented
	// memory address. Otherwise, Offset is the represented offset into
	// Mappable.
	Offset uint64
}

func (k *Key) release() {
	if k.MappingIdentity != nil {
		k.MappingIdentity.DecRef()
	}
	k.Mappable = nil
	k.MappingIdentity = nil
}

func (k *Key) clone() Key {
	if k.MappingIdentity != nil {
		k.MappingIdentity.IncRef()
	}
	return *k
}

// Preconditions: k.Kind == KindPrivate or KindSharedPrivate.
func (k *Key) addr() uintptr {
	return uintptr(k.Offset)
}

// matches returns true if a wakeup on k2 should wake a waiter waiting on k.
func (k *Key) matches(k2 *Key) bool {
	// k.MappingIdentity is ignored; it's only used for reference counting.
	return k.Kind == k2.Kind && k.Mappable == k2.Mappable && k.Offset == k2.Offset
}

// Checker abstracts memory accesses. This is useful because the "addresses"
// used in this package may not be real addresses (they could be indices of an
// array, for example), or they could be mapped via some special mechanism.
//
// TODO: Replace this with usermem.IO.
type Checker interface {
	// Check should validate that given address contains the given value.
	// If it does not contain the value, syserror.EAGAIN must be returned.
	// Any other error may be returned, which will be propagated.
	Check(addr uintptr, val uint32) error

	// Op should atomically perform the operation encoded in op on the data
	// pointed to by addr, then apply the comparison encoded in op to the
	// original value at addr, returning the result.
	// Note that op is an opaque operation whose behaviour is defined
	// outside of the futex manager.
	Op(addr uintptr, op uint32) (bool, error)

	// GetSharedKey returns a Key with kind KindSharedPrivate or
	// KindSharedMappable corresponding to the memory mapped at address addr.
	//
	// If GetSharedKey returns a Key with a non-nil MappingIdentity, a
	// reference is held on the MappingIdentity, which must be dropped by the
	// caller when the Key is no longer in use.
	GetSharedKey(addr uintptr) (Key, error)
}

// Waiter is the struct which gets enqueued into buckets for wake up routines
// and requeue routines to scan and notify. Once a Waiter has been enqueued by
// WaitPrepare(), callers may listen on C for wake up events.
type Waiter struct {
	// Synchronization:
	//
	// - A Waiter that is not enqueued in a bucket is exclusively owned (no
	// synchronization applies).
	//
	// - A Waiter is enqueued in a bucket by calling WaitPrepare(). After this,
	// waiterEntry, bucket, and key are protected by the bucket.mu ("bucket
	// lock") of the containing bucket, and bitmask is immutable. Note that
	// since bucket is mutated using atomic memory operations, bucket.Load()
	// may be called without holding the bucket lock, although it may change
	// racily. See WaitComplete().
	//
	// - A Waiter is only guaranteed to be no longer queued after calling
	// WaitComplete().

	// waiterEntry links Waiter into bucket.waiters.
	waiterEntry

	// bucket is the bucket this waiter is queued in. If bucket is nil, the
	// waiter is not waiting and is not in any bucket.
	bucket AtomicPtrBucket

	// C is sent to when the Waiter is woken.
	C chan struct{}

	// key is what this waiter is waiting on.
	key Key

	// The bitmask we're waiting on.
	// This is used the case of a FUTEX_WAKE_BITSET.
	bitmask uint32
}

// NewWaiter returns a new unqueued Waiter.
func NewWaiter() *Waiter {
	return &Waiter{
		C: make(chan struct{}, 1),
	}
}

// woken returns true if w has been woken since the last call to WaitPrepare.
func (w *Waiter) woken() bool {
	return len(w.C) != 0
}

// bucket holds a list of waiters for a given address hash.
//
// +stateify savable
type bucket struct {
	// mu protects waiters and contained Waiter state. See comment in Waiter.
	mu sync.Mutex `state:"nosave"`

	waiters waiterList `state:"zerovalue"`
}

// wakeLocked wakes up to n waiters matching the bitmask at the addr for this
// bucket and returns the number of waiters woken.
//
// Preconditions: b.mu must be locked.
func (b *bucket) wakeLocked(key *Key, bitmask uint32, n int) int {
	done := 0
	for w := b.waiters.Front(); done < n && w != nil; {
		if !w.key.matches(key) || w.bitmask&bitmask == 0 {
			// Not matching.
			w = w.Next()
			continue
		}

		// Remove from the bucket and wake the waiter.
		woke := w
		w = w.Next() // Next iteration.
		b.waiters.Remove(woke)
		woke.C <- struct{}{}

		// NOTE: The above channel write establishes a write barrier according
		// to the memory model, so nothing may be ordered around it. Since
		// we've dequeued woke and will never touch it again, we can safely
		// store nil to woke.bucket here and allow the WaitComplete() to
		// short-circuit grabbing the bucket lock. If they somehow miss the
		// store, we are still holding the lock, so we can know that they won't
		// dequeue woke, assume it's free and have the below operation
		// afterwards.
		woke.bucket.Store(nil)
		done++
	}
	return done
}

// requeueLocked takes n waiters from the bucket and moves them to naddr on the
// bucket "to".
//
// Preconditions: b and to must be locked.
func (b *bucket) requeueLocked(to *bucket, key, nkey *Key, n int) int {
	done := 0
	for w := b.waiters.Front(); done < n && w != nil; {
		if !w.key.matches(key) {
			// Not matching.
			w = w.Next()
			continue
		}

		requeued := w
		w = w.Next() // Next iteration.
		b.waiters.Remove(requeued)
		requeued.key.release()
		requeued.key = nkey.clone()
		to.waiters.PushBack(requeued)
		requeued.bucket.Store(to)
		done++
	}
	return done
}

const (
	// bucketCount is the number of buckets per Manager. By having many of
	// these we reduce contention when concurrent yet unrelated calls are made.
	bucketCount     = 1 << bucketCountBits
	bucketCountBits = 10
)

// getKey returns a Key representing address addr in c.
func getKey(c Checker, addr uintptr, private bool) (Key, error) {
	// Ensure the address is aligned.
	// It must be a DWORD boundary.
	if addr&0x3 != 0 {
		return Key{}, syserror.EINVAL
	}
	if private {
		return Key{Kind: KindPrivate, Offset: uint64(addr)}, nil
	}
	return c.GetSharedKey(addr)
}

// bucketIndexForAddr returns the index into Manager.buckets for addr.
func bucketIndexForAddr(addr uintptr) uintptr {
	// - The bottom 2 bits of addr must be 0, per getKey.
	//
	// - On amd64, the top 16 bits of addr (bits 48-63) must be equal to bit 47
	// for a canonical address, and (on all existing platforms) bit 47 must be
	// 0 for an application address.
	//
	// Thus 19 bits of addr are "useless" for hashing, leaving only 45 "useful"
	// bits. We choose one of the simplest possible hash functions that at
	// least uses all 45 useful bits in the output, given that bucketCountBits
	// == 10. This hash function also has the property that it will usually map
	// adjacent addresses to adjacent buckets, slightly improving memory
	// locality when an application synchronization structure uses multiple
	// nearby futexes.
	//
	// Note that despite the large number of arithmetic operations in the
	// function, many components can be computed in parallel, such that the
	// critical path is 1 bit shift + 3 additions (2 in h1, then h1 + h2). This
	// is also why h1 and h2 are grouped separately; for "(addr >> 2) + ... +
	// (addr >> 42)" without any additional grouping, the compiler puts all 4
	// additions in the critical path.
	h1 := (addr >> 2) + (addr >> 12) + (addr >> 22)
	h2 := (addr >> 32) + (addr >> 42)
	return (h1 + h2) % bucketCount
}

// Manager holds futex state for a single virtual address space.
//
// +stateify savable
type Manager struct {
	// privateBuckets holds buckets for KindPrivate and KindSharedPrivate
	// futexes.
	privateBuckets [bucketCount]bucket `state:"zerovalue"`

	// sharedBucket is the bucket for KindSharedMappable futexes. sharedBucket
	// may be shared by multiple Managers. The sharedBucket pointer is
	// immutable.
	sharedBucket *bucket
}

// NewManager returns an initialized futex manager.
func NewManager() *Manager {
	return &Manager{
		sharedBucket: &bucket{},
	}
}

// Fork returns a new Manager. Shared futex clients using the returned Manager
// may interoperate with those using m.
func (m *Manager) Fork() *Manager {
	return &Manager{
		sharedBucket: m.sharedBucket,
	}
}

// lockBucket returns a locked bucket for the given key.
func (m *Manager) lockBucket(k *Key) *bucket {
	var b *bucket
	if k.Kind == KindSharedMappable {
		b = m.sharedBucket
	} else {
		b = &m.privateBuckets[bucketIndexForAddr(k.addr())]
	}
	b.mu.Lock()
	return b
}

// lockBuckets returns locked buckets for the given keys.
func (m *Manager) lockBuckets(k1, k2 *Key) (*bucket, *bucket) {
	// Buckets must be consistently ordered to avoid circular lock
	// dependencies. We order buckets in m.privateBuckets by index (lowest
	// index first), and all buckets in m.privateBuckets precede
	// m.sharedBucket.

	// Handle the common case first:
	if k1.Kind != KindSharedMappable && k2.Kind != KindSharedMappable {
		i1 := bucketIndexForAddr(k1.addr())
		i2 := bucketIndexForAddr(k2.addr())
		b1 := &m.privateBuckets[i1]
		b2 := &m.privateBuckets[i2]
		switch {
		case i1 < i2:
			b1.mu.Lock()
			b2.mu.Lock()
		case i2 < i1:
			b2.mu.Lock()
			b1.mu.Lock()
		default:
			b1.mu.Lock()
		}
		return b1, b2
	}

	// At least one of b1 or b2 should be m.sharedBucket.
	b1 := m.sharedBucket
	b2 := m.sharedBucket
	if k1.Kind != KindSharedMappable {
		b1 = m.lockBucket(k1)
	} else if k2.Kind != KindSharedMappable {
		b2 = m.lockBucket(k2)
	}
	m.sharedBucket.mu.Lock()
	return b1, b2
}

// Wake wakes up to n waiters matching the bitmask on the given addr.
// The number of waiters woken is returned.
func (m *Manager) Wake(c Checker, addr uintptr, private bool, bitmask uint32, n int) (int, error) {
	// This function is very hot; avoid defer.
	k, err := getKey(c, addr, private)
	if err != nil {
		return 0, err
	}

	b := m.lockBucket(&k)
	r := b.wakeLocked(&k, bitmask, n)

	b.mu.Unlock()
	k.release()
	return r, nil
}

func (m *Manager) doRequeue(c Checker, addr, naddr uintptr, private bool, checkval bool, val uint32, nwake int, nreq int) (int, error) {
	k1, err := getKey(c, addr, private)
	if err != nil {
		return 0, err
	}
	defer k1.release()
	k2, err := getKey(c, naddr, private)
	if err != nil {
		return 0, err
	}
	defer k2.release()

	b1, b2 := m.lockBuckets(&k1, &k2)
	defer b1.mu.Unlock()
	if b2 != b1 {
		defer b2.mu.Unlock()
	}

	if checkval {
		if err := c.Check(addr, val); err != nil {
			return 0, err
		}
	}

	// Wake the number required.
	done := b1.wakeLocked(&k1, ^uint32(0), nwake)

	// Requeue the number required.
	b1.requeueLocked(b2, &k1, &k2, nreq)

	return done, nil
}

// Requeue wakes up to nwake waiters on the given addr, and unconditionally
// requeues up to nreq waiters on naddr.
func (m *Manager) Requeue(c Checker, addr, naddr uintptr, private bool, nwake int, nreq int) (int, error) {
	return m.doRequeue(c, addr, naddr, private, false, 0, nwake, nreq)
}

// RequeueCmp atomically checks that the addr contains val (via the Checker),
// wakes up to nwake waiters on addr and then unconditionally requeues nreq
// waiters on naddr.
func (m *Manager) RequeueCmp(c Checker, addr, naddr uintptr, private bool, val uint32, nwake int, nreq int) (int, error) {
	return m.doRequeue(c, addr, naddr, private, true, val, nwake, nreq)
}

// WakeOp atomically applies op to the memory address addr2, wakes up to nwake1
// waiters unconditionally from addr1, and, based on the original value at addr2
// and a comparison encoded in op, wakes up to nwake2 waiters from addr2.
// It returns the total number of waiters woken.
func (m *Manager) WakeOp(c Checker, addr1, addr2 uintptr, private bool, nwake1 int, nwake2 int, op uint32) (int, error) {
	k1, err := getKey(c, addr1, private)
	if err != nil {
		return 0, err
	}
	defer k1.release()
	k2, err := getKey(c, addr2, private)
	if err != nil {
		return 0, err
	}
	defer k2.release()

	b1, b2 := m.lockBuckets(&k1, &k2)
	defer b1.mu.Unlock()
	if b2 != b1 {
		defer b2.mu.Unlock()
	}

	done := 0
	cond, err := c.Op(addr2, op)
	if err != nil {
		return 0, err
	}

	// Wake up up to nwake1 entries from the first bucket.
	done = b1.wakeLocked(&k1, ^uint32(0), nwake1)

	// Wake up up to nwake2 entries from the second bucket if the
	// operation yielded true.
	if cond {
		done += b2.wakeLocked(&k2, ^uint32(0), nwake2)
	}

	return done, nil
}

// WaitPrepare atomically checks that addr contains val (via the Checker), then
// enqueues w to be woken by a send to w.C. If WaitPrepare returns nil, the
// Waiter must be subsequently removed by calling WaitComplete, whether or not
// a wakeup is received on w.C.
func (m *Manager) WaitPrepare(w *Waiter, c Checker, addr uintptr, private bool, val uint32, bitmask uint32) error {
	k, err := getKey(c, addr, private)
	if err != nil {
		return err
	}
	// Ownership of k is transferred to w below.

	// Prepare the Waiter before taking the bucket lock.
	select {
	case <-w.C:
	default:
	}
	w.key = k
	w.bitmask = bitmask

	b := m.lockBucket(&k)
	// This function is very hot; avoid defer.

	// Perform our atomic check.
	if err := c.Check(addr, val); err != nil {
		b.mu.Unlock()
		w.key.release()
		return err
	}

	// Add the waiter to the bucket.
	b.waiters.PushBack(w)
	w.bucket.Store(b)

	b.mu.Unlock()
	return nil
}

// WaitComplete must be called when a Waiter previously added by WaitPrepare is
// no longer eligible to be woken.
func (m *Manager) WaitComplete(w *Waiter) {
	// Remove w from the bucket it's in.
	for {
		b := w.bucket.Load()

		// If b is nil, the waiter isn't in any bucket anymore. This can't be
		// racy because the waiter can't be concurrently re-queued in another
		// bucket.
		if b == nil {
			break
		}

		// Take the bucket lock. Note that without holding the bucket lock, the
		// waiter is not guaranteed to stay in that bucket, so after we take
		// the bucket lock, we must ensure that the bucket hasn't changed: if
		// it happens to have changed, we release the old bucket lock and try
		// again with the new bucket; if it hasn't changed, we know it won't
		// change now because we hold the lock.
		b.mu.Lock()
		if b != w.bucket.Load() {
			b.mu.Unlock()
			continue
		}

		// Remove w from b.
		b.waiters.Remove(w)
		w.bucket.Store(nil)
		b.mu.Unlock()
		break
	}

	// Release references held by the waiter.
	w.key.release()
}