1 files changed, 405 insertions, 0 deletions
diff --git a/pkg/sentry/kernel/futex/futex.go b/pkg/sentry/kernel/futex/futex.go
new file mode 100644
index 000000000..b3ba57a2c
--- /dev/null
+++ b/pkg/sentry/kernel/futex/futex.go
@@ -0,0 +1,405 @@
+// Copyright 2018 Google Inc.
+//
+// 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"
+	"sync/atomic"
+
+	"gvisor.googlesource.com/gvisor/pkg/syserror"
+)
+
+// 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)
+}
+
+// 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, complete, and addr are protected by the bucket.mu ("bucket
+	// lock") of the containing bucket, and bitmask is immutable. complete and
+	// addr are additionally mutated using atomic memory operations, ensuring
+	// that they can be read using atomic memory operations without holding the
+	// bucket lock.
+	//
+	// - A Waiter is only guaranteed to be no longer queued after calling
+	// WaitComplete().
+
+	// waiterEntry links Waiter into bucket.waiters.
+	waiterEntry
+
+	// complete is 1 if the Waiter was removed from its bucket by a wakeup and
+	// 0 otherwise.
+	complete int32
+
+	// C is sent to when the Waiter is woken.
+	C chan struct{}
+
+	// addr is the address being waited on.
+	addr uintptr
+
+	// 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),
+	}
+}
+
+// bucket holds a list of waiters for a given address hash.
+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(addr uintptr, bitmask uint32, n int) int {
+	done := 0
+	for w := b.waiters.Front(); done < n && w != nil; {
+		if w.addr != addr || 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 w and will never touch it
+		// again, we can safely store 1 to w.complete here and allow
+		// the WaitComplete() to short-circuit grabbing the bucket
+		// lock. If they somehow miss the w.complete, we are still
+		// holding the lock, so we can know that they won't dequeue w,
+		// assume it's free and have the below operation afterwards.
+		atomic.StoreInt32(&woke.complete, 1)
+		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, addr, naddr uintptr, n int) int {
+	done := 0
+	for w := b.waiters.Front(); done < n && w != nil; {
+		if w.addr != addr {
+			// Not matching.
+			w = w.Next()
+			continue
+		}
+
+		requeued := w
+		w = w.Next() // Next iteration.
+		b.waiters.Remove(requeued)
+		atomic.StoreUintptr(&requeued.addr, naddr)
+		to.waiters.PushBack(requeued)
+		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
+)
+
+func checkAddr(addr uintptr) error {
+	// Ensure the address is aligned.
+	// It must be a DWORD boundary.
+	if addr&0x3 != 0 {
+		return syserror.EINVAL
+	}
+
+	return nil
+}
+
+// bucketIndexForAddr returns the index into Manager.buckets for addr.
+func bucketIndexForAddr(addr uintptr) uintptr {
+	// - The bottom 2 bits of addr must be 0, per checkAddr.
+	//
+	// - 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.
+type Manager struct {
+	buckets [bucketCount]bucket
+}
+
+// NewManager returns an initialized futex manager.
+// N.B. we use virtual address to tag futexes, so it only works for private
+// (within a single process) futex.
+func NewManager() *Manager {
+	return &Manager{}
+}
+
+// lockBucket returns a locked bucket for the given addr.
+//
+// Preconditions: checkAddr(addr) == nil.
+func (m *Manager) lockBucket(addr uintptr) *bucket {
+	b := &m.buckets[bucketIndexForAddr(addr)]
+	b.mu.Lock()
+	return b
+}
+
+// lockBuckets returns locked buckets for the given addrs.
+//
+// Preconditions: checkAddr(addr1) == checkAddr(addr2) == nil.
+func (m *Manager) lockBuckets(addr1 uintptr, addr2 uintptr) (*bucket, *bucket) {
+	i1 := bucketIndexForAddr(addr1)
+	i2 := bucketIndexForAddr(addr2)
+	b1 := &m.buckets[i1]
+	b2 := &m.buckets[i2]
+
+	// Ensure that buckets are locked in a consistent order (lowest index
+	// first) to avoid circular locking.
+	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
+}
+
+// Wake wakes up to n waiters matching the bitmask on the given addr.
+// The number of waiters woken is returned.
+func (m *Manager) Wake(addr uintptr, bitmask uint32, n int) (int, error) {
+	if err := checkAddr(addr); err != nil {
+		return 0, err
+	}
+
+	b := m.lockBucket(addr)
+	// This function is very hot; avoid defer.
+	r := b.wakeLocked(addr, bitmask, n)
+	b.mu.Unlock()
+	return r, nil
+}
+
+func (m *Manager) doRequeue(c Checker, addr uintptr, val uint32, naddr uintptr, nwake int, nreq int) (int, error) {
+	if err := checkAddr(addr); err != nil {
+		return 0, err
+	}
+	if err := checkAddr(naddr); err != nil {
+		return 0, err
+	}
+
+	b1, b2 := m.lockBuckets(addr, naddr)
+	defer b1.mu.Unlock()
+	if b2 != b1 {
+		defer b2.mu.Unlock()
+	}
+
+	// Check our value.
+	// This only applied for RequeueCmp().
+	if c != nil {
+		if err := c.Check(addr, val); err != nil {
+			return 0, err
+		}
+	}
+
+	// Wake the number required.
+	done := b1.wakeLocked(addr, ^uint32(0), nwake)
+
+	// Requeue the number required.
+	b1.requeueLocked(b2, addr, naddr, 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(addr uintptr, naddr uintptr, nwake int, nreq int) (int, error) {
+	return m.doRequeue(nil, addr, 0, naddr, 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 uintptr, val uint32, naddr uintptr, nwake int, nreq int) (int, error) {
+	return m.doRequeue(c, addr, val, naddr, 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 uintptr, addr2 uintptr, nwake1 int, nwake2 int, op uint32) (int, error) {
+	if err := checkAddr(addr1); err != nil {
+		return 0, err
+	}
+	if err := checkAddr(addr2); err != nil {
+		return 0, err
+	}
+
+	b1, b2 := m.lockBuckets(addr1, addr2)
+
+	done := 0
+	cond, err := c.Op(addr2, op)
+	if err == nil {
+		// Wake up up to nwake1 entries from the first bucket.
+		done = b1.wakeLocked(addr1, ^uint32(0), nwake1)
+
+		// Wake up up to nwake2 entries from the second bucket if the
+		// operation yielded true.
+		if cond {
+			done += b2.wakeLocked(addr2, ^uint32(0), nwake2)
+		}
+	}
+
+	b1.mu.Unlock()
+	if b2 != b1 {
+		b2.mu.Unlock()
+	}
+	return done, err
+}
+
+// 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, val uint32, bitmask uint32) error {
+	if err := checkAddr(addr); err != nil {
+		return err
+	}
+
+	// Prepare the Waiter before taking the bucket lock.
+	w.complete = 0
+	select {
+	case <-w.C:
+	default:
+	}
+	w.addr = addr
+	w.bitmask = bitmask
+
+	b := m.lockBucket(addr)
+	// This function is very hot; avoid defer.
+
+	// Perform our atomic check.
+	if err := c.Check(addr, val); err != nil {
+		b.mu.Unlock()
+		return err
+	}
+
+	// Add the waiter to the bucket.
+	b.waiters.PushBack(w)
+
+	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) {
+	// Can we short-circuit acquiring the lock?
+	// This is the happy path where a notification
+	// was received and we don't need to dequeue this
+	// waiter from any list (or take any locks).
+	if atomic.LoadInt32(&w.complete) != 0 {
+		return
+	}
+
+	// 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.
+	var b *bucket
+	for {
+		addr := atomic.LoadUintptr(&w.addr)
+		b = m.lockBucket(addr)
+		// We still have to use an atomic load here, because if w was racily
+		// requeued then w.addr is not protected by b.mu.
+		if addr == atomic.LoadUintptr(&w.addr) {
+			break
+		}
+		b.mu.Unlock()
+	}
+
+	// Remove waiter from the bucket. w.complete can only be stored with b.mu
+	// locked, so this load doesn't need to use sync/atomic.
+	if w.complete == 0 {
+		b.waiters.Remove(w)
+	}
+	b.mu.Unlock()
+}