diff --git a/README.md b/README.md
index 9608fa7..6f170c3 100644
--- a/README.md
+++ b/README.md
@@ -72,10 +72,10 @@ Performance is tested by running
   - [ ] session extension
 - [ ] custom SQL functions
 - [ ] custom VFSes
-  - [ ] in-memory VFS
+  - [x] custom VFS API
+  - [x] in-memory VFS
   - [x] read-only VFS, wrapping an [`io.ReaderAt`](https://pkg.go.dev/io#ReaderAt)
   - [ ] cloud-based VFS, based on [Cloud Backed SQLite](https://sqlite.org/cloudsqlite/doc/trunk/www/index.wiki)
-  - [x] custom VFS API
 
 ### Alternatives
 
diff --git a/sqlite3vfs/lock.go b/sqlite3vfs/lock.go
index 496622c..e05580a 100644
--- a/sqlite3vfs/lock.go
+++ b/sqlite3vfs/lock.go
@@ -14,73 +14,73 @@ const (
 	_SHARED_SIZE   = 510
 )
 
-func (file *vfsFile) Lock(eLock LockLevel) error {
+func (f *vfsFile) Lock(lock LockLevel) error {
 	// Argument check. SQLite never explicitly requests a pending lock.
-	if eLock != LOCK_SHARED && eLock != LOCK_RESERVED && eLock != LOCK_EXCLUSIVE {
+	if lock != LOCK_SHARED && lock != LOCK_RESERVED && lock != LOCK_EXCLUSIVE {
 		panic(util.AssertErr())
 	}
 
 	switch {
-	case file.lock < LOCK_NONE || file.lock > LOCK_EXCLUSIVE:
+	case f.lock < LOCK_NONE || f.lock > LOCK_EXCLUSIVE:
 		// Connection state check.
 		panic(util.AssertErr())
-	case file.lock == LOCK_NONE && eLock > LOCK_SHARED:
+	case f.lock == LOCK_NONE && lock > LOCK_SHARED:
 		// We never move from unlocked to anything higher than a shared lock.
 		panic(util.AssertErr())
-	case file.lock != LOCK_SHARED && eLock == LOCK_RESERVED:
+	case f.lock != LOCK_SHARED && lock == LOCK_RESERVED:
 		// A shared lock is always held when a reserved lock is requested.
 		panic(util.AssertErr())
 	}
 
 	// If we already have an equal or more restrictive lock, do nothing.
-	if file.lock >= eLock {
+	if f.lock >= lock {
 		return nil
 	}
 
 	// Do not allow any kind of write-lock on a read-only database.
-	if file.readOnly && eLock >= LOCK_RESERVED {
+	if f.readOnly && lock >= LOCK_RESERVED {
 		return _IOERR_LOCK
 	}
 
-	switch eLock {
+	switch lock {
 	case LOCK_SHARED:
 		// Must be unlocked to get SHARED.
-		if file.lock != LOCK_NONE {
+		if f.lock != LOCK_NONE {
 			panic(util.AssertErr())
 		}
-		if rc := osGetSharedLock(file.File, file.lockTimeout); rc != _OK {
+		if rc := osGetSharedLock(f.File, f.lockTimeout); rc != _OK {
 			return rc
 		}
-		file.lock = LOCK_SHARED
+		f.lock = LOCK_SHARED
 		return nil
 
 	case LOCK_RESERVED:
 		// Must be SHARED to get RESERVED.
-		if file.lock != LOCK_SHARED {
+		if f.lock != LOCK_SHARED {
 			panic(util.AssertErr())
 		}
-		if rc := osGetReservedLock(file.File, file.lockTimeout); rc != _OK {
+		if rc := osGetReservedLock(f.File, f.lockTimeout); rc != _OK {
 			return rc
 		}
-		file.lock = LOCK_RESERVED
+		f.lock = LOCK_RESERVED
 		return nil
 
 	case LOCK_EXCLUSIVE:
 		// Must be SHARED, RESERVED or PENDING to get EXCLUSIVE.
-		if file.lock <= LOCK_NONE || file.lock >= LOCK_EXCLUSIVE {
+		if f.lock <= LOCK_NONE || f.lock >= LOCK_EXCLUSIVE {
 			panic(util.AssertErr())
 		}
 		// A PENDING lock is needed before acquiring an EXCLUSIVE lock.
-		if file.lock < LOCK_PENDING {
-			if rc := osGetPendingLock(file.File); rc != _OK {
+		if f.lock < LOCK_PENDING {
+			if rc := osGetPendingLock(f.File); rc != _OK {
 				return rc
 			}
-			file.lock = LOCK_PENDING
+			f.lock = LOCK_PENDING
 		}
-		if rc := osGetExclusiveLock(file.File, file.lockTimeout); rc != _OK {
+		if rc := osGetExclusiveLock(f.File, f.lockTimeout); rc != _OK {
 			return rc
 		}
-		file.lock = LOCK_EXCLUSIVE
+		f.lock = LOCK_EXCLUSIVE
 		return nil
 
 	default:
@@ -88,33 +88,33 @@ func (file *vfsFile) Lock(eLock LockLevel) error {
 	}
 }
 
-func (file *vfsFile) Unlock(eLock LockLevel) error {
+func (f *vfsFile) Unlock(lock LockLevel) error {
 	// Argument check.
-	if eLock != LOCK_NONE && eLock != LOCK_SHARED {
+	if lock != LOCK_NONE && lock != LOCK_SHARED {
 		panic(util.AssertErr())
 	}
 
 	// Connection state check.
-	if file.lock < LOCK_NONE || file.lock > LOCK_EXCLUSIVE {
+	if f.lock < LOCK_NONE || f.lock > LOCK_EXCLUSIVE {
 		panic(util.AssertErr())
 	}
 
 	// If we don't have a more restrictive lock, do nothing.
-	if file.lock <= eLock {
+	if f.lock <= lock {
 		return nil
 	}
 
-	switch eLock {
+	switch lock {
 	case LOCK_SHARED:
-		if rc := osDowngradeLock(file.File, file.lock); rc != _OK {
+		if rc := osDowngradeLock(f.File, f.lock); rc != _OK {
 			return rc
 		}
-		file.lock = LOCK_SHARED
+		f.lock = LOCK_SHARED
 		return nil
 
 	case LOCK_NONE:
-		rc := osReleaseLock(file.File, file.lock)
-		file.lock = LOCK_NONE
+		rc := osReleaseLock(f.File, f.lock)
+		f.lock = LOCK_NONE
 		return rc
 
 	default:
@@ -122,16 +122,16 @@ func (file *vfsFile) Unlock(eLock LockLevel) error {
 	}
 }
 
-func (file *vfsFile) CheckReservedLock() (bool, error) {
+func (f *vfsFile) CheckReservedLock() (bool, error) {
 	// Connection state check.
-	if file.lock < LOCK_NONE || file.lock > LOCK_EXCLUSIVE {
+	if f.lock < LOCK_NONE || f.lock > LOCK_EXCLUSIVE {
 		panic(util.AssertErr())
 	}
 
-	if file.lock >= LOCK_RESERVED {
+	if f.lock >= LOCK_RESERVED {
 		return true, nil
 	}
-	return osCheckReservedLock(file.File)
+	return osCheckReservedLock(f.File)
 }
 
 func osGetReservedLock(file *os.File, timeout time.Duration) _ErrorCode {
diff --git a/sqlite3vfs/memory.go b/sqlite3vfs/memory.go
index f508e4e..b6e114c 100644
--- a/sqlite3vfs/memory.go
+++ b/sqlite3vfs/memory.go
@@ -2,7 +2,9 @@ package sqlite3vfs
 
 import (
 	"io"
+	"runtime"
 	"sync"
+	"time"
 )
 
 // A MemoryVFS is a [VFS] for memory databases.
@@ -42,14 +44,19 @@ func (vfs MemoryVFS) FullPathname(name string) (string, error) {
 const memSectorSize = 65536
 
 type MemoryDB struct {
+	mtx  sync.RWMutex
 	size int64
 	data []*[memSectorSize]byte
-	mtx  sync.Mutex
+
+	locker   sync.Mutex
+	pending  *memoryFile
+	reserved *memoryFile
+	shared   int
 }
 
 type memoryFile struct {
 	*MemoryDB
-	locked   bool
+	lock     LockLevel
 	readOnly bool
 }
 
@@ -58,6 +65,9 @@ func (m *memoryFile) Close() error {
 }
 
 func (m *memoryFile) ReadAt(b []byte, off int64) (n int, err error) {
+	m.mtx.RLock()
+	defer m.mtx.RUnlock()
+
 	if off >= m.size {
 		return 0, io.EOF
 	}
@@ -71,6 +81,9 @@ func (m *memoryFile) ReadAt(b []byte, off int64) (n int, err error) {
 }
 
 func (m *memoryFile) WriteAt(b []byte, off int64) (n int, err error) {
+	m.mtx.Lock()
+	defer m.mtx.Unlock()
+
 	base := off / memSectorSize
 	rest := off % memSectorSize
 	if base >= int64(len(m.data)) {
@@ -84,6 +97,12 @@ func (m *memoryFile) WriteAt(b []byte, off int64) (n int, err error) {
 }
 
 func (m *memoryFile) Truncate(size int64) error {
+	m.mtx.Lock()
+	defer m.mtx.Unlock()
+	return m.truncate(size)
+}
+
+func (m *memoryFile) truncate(size int64) error {
 	if size < m.size {
 		base := size / memSectorSize
 		rest := size % memSectorSize
@@ -107,37 +126,93 @@ func (*memoryFile) Sync(flag SyncFlag) error {
 }
 
 func (m *memoryFile) Size() (int64, error) {
+	m.mtx.RLock()
+	defer m.mtx.RUnlock()
 	return m.size, nil
 }
 
 func (m *memoryFile) Lock(lock LockLevel) error {
+	if m.lock >= lock {
+		return nil
+	}
+
 	if m.readOnly && lock >= LOCK_RESERVED {
 		return _IOERR_LOCK
 	}
-	if m.locked || m.mtx.TryLock() {
-		m.locked = true
-		return nil
+
+	m.locker.Lock()
+	defer m.locker.Unlock()
+	deadline := time.Now().Add(time.Millisecond)
+
+	switch lock {
+	case LOCK_SHARED:
+		for m.pending != nil {
+			if time.Now().After(deadline) {
+				return _BUSY
+			}
+			m.locker.Unlock()
+			runtime.Gosched()
+			m.locker.Lock()
+		}
+		m.shared++
+
+	case LOCK_RESERVED:
+		if m.reserved != nil {
+			return _BUSY
+		}
+		m.reserved = m
+
+	case LOCK_EXCLUSIVE:
+		if m.lock < LOCK_PENDING {
+			if m.pending != nil {
+				return _BUSY
+			}
+			m.lock = LOCK_PENDING
+			m.pending = m
+		}
+
+		for m.shared > 1 {
+			if time.Now().After(deadline) {
+				return _BUSY
+			}
+			m.locker.Unlock()
+			runtime.Gosched()
+			m.locker.Lock()
+		}
 	}
-	return _BUSY
+
+	m.lock = lock
+	return nil
 }
 
 func (m *memoryFile) Unlock(lock LockLevel) error {
-	if m.locked && lock == LOCK_NONE {
-		m.locked = false
-		m.mtx.Unlock()
+	if m.lock <= lock {
+		return nil
 	}
+
+	m.locker.Lock()
+	defer m.locker.Unlock()
+
+	if m.pending == m {
+		m.pending = nil
+	}
+	if m.reserved == m {
+		m.reserved = nil
+	}
+	if lock < LOCK_SHARED {
+		m.shared--
+	}
+	m.lock = lock
 	return nil
 }
 
 func (m *memoryFile) CheckReservedLock() (bool, error) {
-	if m.locked {
+	if m.lock >= LOCK_RESERVED {
 		return true, nil
 	}
-	if m.mtx.TryLock() {
-		m.mtx.Unlock()
-		return true, nil
-	}
-	return false, nil
+	m.locker.Lock()
+	defer m.locker.Unlock()
+	return m.reserved != nil, nil
 }
 
 func (*memoryFile) SectorSize() int {
@@ -152,12 +227,18 @@ func (*memoryFile) DeviceCharacteristics() DeviceCharacteristic {
 }
 
 func (m *memoryFile) SizeHint(size int64) error {
+	m.mtx.Lock()
+	defer m.mtx.Unlock()
 	if size > m.size {
-		return m.Truncate(size)
+		return m.truncate(size)
 	}
 	return nil
 }
 
+func (m *memoryFile) LockState() LockLevel {
+	return m.lock
+}
+
 func clear(b []byte) {
 	for i := range b {
 		b[i] = 0