SI-6240 introduces GIL to Scala reflection

On a serious note, I feel really uncomfortable about having to juggle
this slew of locks. Despite that I can't immediately find a deadlock,
I'm 100% sure there is one hiding in the shadows. Hence, I'm abandoning
all runtime reflection locks in favor of a single per-universe one.

10 files changed, 226 insertions, 111 deletions

diff --git a/src/reflect/scala/reflect/internal/BaseTypeSeqs.scala b/src/reflect/scala/reflect/internal/BaseTypeSeqs.scala
index 05aaa462c4..19c67879f5 100644
--- a/src/reflect/scala/reflect/internal/BaseTypeSeqs.scala
+++ b/src/reflect/scala/reflect/internal/BaseTypeSeqs.scala
@@ -38,7 +38,7 @@ trait BaseTypeSeqs {
    *  This is necessary because when run from reflection every base type sequence needs to have a
    *  SynchronizedBaseTypeSeq as mixin.
    */
-  class BaseTypeSeq protected[BaseTypeSeqs] (private[BaseTypeSeqs] val parents: List[Type], private[BaseTypeSeqs] val elems: Array[Type]) {
+  class BaseTypeSeq protected[reflect] (private[BaseTypeSeqs] val parents: List[Type], private[BaseTypeSeqs] val elems: Array[Type]) {
   self =>
     if (Statistics.canEnable) Statistics.incCounter(baseTypeSeqCount)
     if (Statistics.canEnable) Statistics.incCounter(baseTypeSeqLenTotal, elems.length)
diff --git a/src/reflect/scala/reflect/runtime/Gil.scala b/src/reflect/scala/reflect/runtime/Gil.scala
new file mode 100644
index 0000000000..8a8bfebf8c
--- /dev/null
+++ b/src/reflect/scala/reflect/runtime/Gil.scala
@@ -0,0 +1,22 @@
+package scala.reflect
+package runtime
+
+private[reflect] trait Gil {
+  self: SymbolTable =>
+
+  // fixme... please...
+  // there are the following avenues of optimization we discussed with Roland:
+  // 1) replace PackageScope locks with ConcurrentHashMap, because PackageScope materializers seem to be idempotent
+  // 2) unlock unpickling completers by verifying that they are idempotent or moving non-idempotent parts
+  // 3) remove the necessity in global state for isSubType
+  lazy val gil = new java.util.concurrent.locks.ReentrantLock
+
+  @inline final def gilSynchronized[T](body: => T): T = {
+    try {
+      gil.lock()
+      body
+    } finally {
+      gil.unlock()
+    }
+  }
+}
diff --git a/src/reflect/scala/reflect/runtime/JavaMirrors.scala b/src/reflect/scala/reflect/runtime/JavaMirrors.scala
index 487da2eb4b..7ca73505b7 100644
--- a/src/reflect/scala/reflect/runtime/JavaMirrors.scala
+++ b/src/reflect/scala/reflect/runtime/JavaMirrors.scala
@@ -22,7 +22,7 @@ import ReflectionUtils.{staticSingletonInstance, innerSingletonInstance, scalacS
 import scala.language.existentials
 import scala.runtime.{ScalaRunTime, BoxesRunTime}
 
-private[reflect] trait JavaMirrors extends internal.SymbolTable with api.JavaUniverse { thisUniverse: SymbolTable =>
+private[reflect] trait JavaMirrors extends internal.SymbolTable with api.JavaUniverse with TwoWayCaches { thisUniverse: SymbolTable =>
 
   private lazy val mirrors = new WeakHashMap[ClassLoader, WeakReference[JavaMirror]]()
 
@@ -44,9 +44,11 @@ private[reflect] trait JavaMirrors extends internal.SymbolTable with api.JavaUni
 
   trait JavaClassCompleter extends FlagAssigningCompleter
 
-  def runtimeMirror(cl: ClassLoader): Mirror = mirrors get cl match {
-    case Some(WeakReference(m)) => m
-    case _ => createMirror(rootMirror.RootClass, cl)
+  def runtimeMirror(cl: ClassLoader): Mirror = gilSynchronized {
+    mirrors get cl match {
+      case Some(WeakReference(m)) => m
+      case _ => createMirror(rootMirror.RootClass, cl)
+    }
   }
 
   /** The API of a mirror for a reflective universe */
@@ -684,7 +686,7 @@ private[reflect] trait JavaMirrors extends internal.SymbolTable with api.JavaUni
         completeRest()
       }
 
-      def completeRest(): Unit = thisUniverse.synchronized {
+      def completeRest(): Unit = gilSynchronized {
         val tparams = clazz.rawInfo.typeParams
 
         val parents = try {
@@ -889,7 +891,7 @@ private[reflect] trait JavaMirrors extends internal.SymbolTable with api.JavaUni
      * The Scala package with given fully qualified name. Unlike `packageNameToScala`,
      *  this one bypasses the cache.
      */
-    private[JavaMirrors] def makeScalaPackage(fullname: String): ModuleSymbol = {
+    private[JavaMirrors] def makeScalaPackage(fullname: String): ModuleSymbol = gilSynchronized {
       val split = fullname lastIndexOf '.'
       val ownerModule: ModuleSymbol =
         if (split > 0) packageNameToScala(fullname take split) else this.RootPackage
diff --git a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
index 856a9ad8f9..70126e713a 100644
--- a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
+++ b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
@@ -27,11 +27,10 @@ trait JavaUniverseForce { self: runtime.JavaUniverse  =>
 
     this.settings
     this.treeInfo
+    this.gil
     // inaccessible: this.uniqueLock
-    // inaccessible: this.subsametypeLock
-    // inaccessible: this.lubglbLock
-    // inaccessible: this.indentLock
-    // inaccessible: this.toStringLock
+    // inaccessible: this.nextIdLock
+    // inaccessible: this.freshExistentialNameLock
     // inaccessible: this.mirrors
     this.rootMirror
     this.treeBuild
diff --git a/src/reflect/scala/reflect/runtime/SymbolLoaders.scala b/src/reflect/scala/reflect/runtime/SymbolLoaders.scala
index 706572ba2b..30a3855d70 100644
--- a/src/reflect/scala/reflect/runtime/SymbolLoaders.scala
+++ b/src/reflect/scala/reflect/runtime/SymbolLoaders.scala
@@ -67,6 +67,25 @@ private[reflect] trait SymbolLoaders { self: SymbolTable =>
     }
   }
 
+
+  // Since runtime reflection doesn't have a luxury of enumerating all classes
+  // on the classpath, it has to materialize symbols for top-level definitions
+  // (packages, classes, objects) on demand.
+  //
+  // Someone asks us for a class named `foo.Bar`? Easy. Let's speculatively create
+  // a package named `foo` and then look up `newTypeName("bar")` in its decls.
+  // This lookup, implemented in `SymbolLoaders.PackageScope` tests the waters by
+  // trying to to `Class.forName("foo.Bar")` and then creates a ClassSymbol upon
+  // success (the whole story is a bit longer, but the rest is irrelevant here).
+  //
+  // That's all neat, but these non-deterministic mutations of the global symbol
+  // table give a lot of trouble in multi-threaded setting. One of the popular
+  // reflection crashes happens when multiple threads happen to trigger symbol
+  // materialization multiple times for the same symbol, making subsequent
+  // reflective operations stumble upon outrageous stuff like overloaded packages.
+  //
+  // Short of significantly changing SymbolLoaders I see no other way than just
+  // to slap a global lock on materialization in runtime reflection.
   class PackageScope(pkgClass: Symbol) extends Scope(initFingerPrints = -1L) // disable fingerprinting as we do not know entries beforehand
       with SynchronizedScope {
     assert(pkgClass.isType)
@@ -89,9 +108,11 @@ private[reflect] trait SymbolLoaders { self: SymbolTable =>
       else existing.sym.asInstanceOf[T]
     }
 
-    // disable fingerprinting as we do not know entries beforehand
-    private val negatives = mutable.Set[Name]() // Syncnote: Performance only, so need not be protected.
-    override def lookupEntry(name: Name): ScopeEntry = {
+    // package scopes need to synchronize on the GIL
+    // because lookupEntry might cause changes to the global symbol table
+    override def syncLockSynchronized[T](body: => T): T = gilSynchronized(body)
+    private val negatives = new mutable.HashSet[Name]
+    override def lookupEntry(name: Name): ScopeEntry = syncLockSynchronized {
       val e = super.lookupEntry(name)
       if (e != null)
         e
diff --git a/src/reflect/scala/reflect/runtime/SymbolTable.scala b/src/reflect/scala/reflect/runtime/SymbolTable.scala
index bcd4d16cde..7630fffd16 100644
--- a/src/reflect/scala/reflect/runtime/SymbolTable.scala
+++ b/src/reflect/scala/reflect/runtime/SymbolTable.scala
@@ -9,7 +9,7 @@ import scala.reflect.internal.Flags._
  *  It can be used either from a reflexive universe (class scala.reflect.runtime.JavaUniverse), or else from
  *  a runtime compiler that uses reflection to get a class information (class scala.tools.reflect.ReflectGlobal)
  */
-private[scala] trait SymbolTable extends internal.SymbolTable with JavaMirrors with SymbolLoaders with SynchronizedOps {
+private[scala] trait SymbolTable extends internal.SymbolTable with JavaMirrors with SymbolLoaders with SynchronizedOps with Gil {
 
   def info(msg: => String) =
     if (settings.verbose) println("[reflect-compiler] "+msg)
diff --git a/src/reflect/scala/reflect/runtime/SynchronizedOps.scala b/src/reflect/scala/reflect/runtime/SynchronizedOps.scala
index 6aa47a0405..21df973297 100644
--- a/src/reflect/scala/reflect/runtime/SynchronizedOps.scala
+++ b/src/reflect/scala/reflect/runtime/SynchronizedOps.scala
@@ -14,20 +14,25 @@ private[reflect] trait SynchronizedOps extends internal.SymbolTable
 // BaseTypeSeqs
 
   override protected def newBaseTypeSeq(parents: List[Type], elems: Array[Type]) =
-    new BaseTypeSeq(parents, elems) with SynchronizedBaseTypeSeq
+    // only need to synchronize BaseTypeSeqs if they contain refined types
+    if (elems.filter(_.isInstanceOf[RefinedType]).nonEmpty) new BaseTypeSeq(parents, elems) with SynchronizedBaseTypeSeq
+    else new BaseTypeSeq(parents, elems)
 
   trait SynchronizedBaseTypeSeq extends BaseTypeSeq {
-    override def apply(i: Int): Type = synchronized { super.apply(i) }
-    override def rawElem(i: Int) = synchronized { super.rawElem(i) }
-    override def typeSymbol(i: Int): Symbol = synchronized { super.typeSymbol(i) }
-    override def toList: List[Type] = synchronized { super.toList }
-    override def copy(head: Type, offset: Int): BaseTypeSeq = synchronized { super.copy(head, offset) }
-    override def map(f: Type => Type): BaseTypeSeq = synchronized { super.map(f) }
-    override def exists(p: Type => Boolean): Boolean = synchronized { super.exists(p) }
-    override lazy val maxDepth = synchronized { maxDepthOfElems }
-    override def toString = synchronized { super.toString }
-
-    override def lateMap(f: Type => Type): BaseTypeSeq = new MappedBaseTypeSeq(this, f) with SynchronizedBaseTypeSeq
+    override def apply(i: Int): Type = gilSynchronized { super.apply(i) }
+    override def rawElem(i: Int) = gilSynchronized { super.rawElem(i) }
+    override def typeSymbol(i: Int): Symbol = gilSynchronized { super.typeSymbol(i) }
+    override def toList: List[Type] = gilSynchronized { super.toList }
+    override def copy(head: Type, offset: Int): BaseTypeSeq = gilSynchronized { super.copy(head, offset) }
+    override def map(f: Type => Type): BaseTypeSeq = gilSynchronized { super.map(f) }
+    override def exists(p: Type => Boolean): Boolean = gilSynchronized { super.exists(p) }
+    override lazy val maxDepth = gilSynchronized { maxDepthOfElems }
+    override def toString = gilSynchronized { super.toString }
+
+    override def lateMap(f: Type => Type): BaseTypeSeq =
+      // only need to synchronize BaseTypeSeqs if they contain refined types
+      if (map(f).toList.filter(_.isInstanceOf[RefinedType]).nonEmpty) new MappedBaseTypeSeq(this, f) with SynchronizedBaseTypeSeq
+      else new MappedBaseTypeSeq(this, f)
   }
 
 // Scopes
@@ -36,15 +41,19 @@ private[reflect] trait SynchronizedOps extends internal.SymbolTable
   override def newNestedScope(outer: Scope): Scope = new Scope(outer) with SynchronizedScope
 
   trait SynchronizedScope extends Scope {
-    override def isEmpty: Boolean = synchronized { super.isEmpty }
-    override def size: Int = synchronized { super.size }
-    override def enter[T <: Symbol](sym: T): T = synchronized { super.enter(sym) }
-    override def rehash(sym: Symbol, newname: Name) = synchronized { super.rehash(sym, newname) }
-    override def unlink(e: ScopeEntry) = synchronized { super.unlink(e) }
-    override def unlink(sym: Symbol) = synchronized { super.unlink(sym) }
-    override def lookupAll(name: Name) = synchronized { super.lookupAll(name) }
-    override def lookupEntry(name: Name) = synchronized { super.lookupEntry(name) }
-    override def lookupNextEntry(entry: ScopeEntry) = synchronized { super.lookupNextEntry(entry) }
-    override def toList: List[Symbol] = synchronized { super.toList }
+    // we can keep this lock fine-grained, because methods of Scope don't do anything extraordinary, which makes deadlocks impossible
+    // fancy subclasses of internal.Scopes#Scope should do synchronization themselves (e.g. see PackageScope for an example)
+    private lazy val syncLock = new Object
+    def syncLockSynchronized[T](body: => T): T = syncLock.synchronized { body }
+    override def isEmpty: Boolean = syncLockSynchronized { super.isEmpty }
+    override def size: Int = syncLockSynchronized { super.size }
+    override def enter[T <: Symbol](sym: T): T = syncLockSynchronized { super.enter(sym) }
+    override def rehash(sym: Symbol, newname: Name) = syncLockSynchronized { super.rehash(sym, newname) }
+    override def unlink(e: ScopeEntry) = syncLockSynchronized { super.unlink(e) }
+    override def unlink(sym: Symbol) = syncLockSynchronized { super.unlink(sym) }
+    override def lookupAll(name: Name) = syncLockSynchronized { super.lookupAll(name) }
+    override def lookupEntry(name: Name) = syncLockSynchronized { super.lookupEntry(name) }
+    override def lookupNextEntry(entry: ScopeEntry) = syncLockSynchronized { super.lookupNextEntry(entry) }
+    override def toList: List[Symbol] = syncLockSynchronized { super.toList }
   }
 }
diff --git a/src/reflect/scala/reflect/runtime/SynchronizedSymbols.scala b/src/reflect/scala/reflect/runtime/SynchronizedSymbols.scala
index c0b4261d11..a52921c5c6 100644
--- a/src/reflect/scala/reflect/runtime/SynchronizedSymbols.scala
+++ b/src/reflect/scala/reflect/runtime/SynchronizedSymbols.scala
@@ -6,14 +6,18 @@ import scala.reflect.io.AbstractFile
 
 private[reflect] trait SynchronizedSymbols extends internal.Symbols { self: SymbolTable =>
 
-  override protected def nextId() = synchronized { super.nextId() }
+  // we can keep this lock fine-grained, because nextId is just a simple increment, which makes deadlocks impossible
+  private lazy val nextIdLock = new Object
+  override protected def nextId() = nextIdLock.synchronized { super.nextId() }
 
+  // we can keep this lock fine-grained, because freshExistentialName is just a simple increment, which makes deadlocks impossible
+  private lazy val freshExistentialNameLock = new Object
   override protected def freshExistentialName(suffix: String) =
-    synchronized { super.freshExistentialName(suffix) }
+    freshExistentialNameLock.synchronized { super.freshExistentialName(suffix) }
 
   // Set the fields which point companions at one another.  Returns the module.
   override def connectModuleToClass(m: ModuleSymbol, moduleClass: ClassSymbol): ModuleSymbol =
-    synchronized { super.connectModuleToClass(m, moduleClass) }
+    gilSynchronized { super.connectModuleToClass(m, moduleClass) }
 
   override def newFreeTermSymbol(name: TermName, value: => Any, flags: Long = 0L, origin: String = null): FreeTermSymbol =
     new FreeTermSymbol(name, value, origin) with SynchronizedTermSymbol initFlags flags
@@ -25,27 +29,27 @@ private[reflect] trait SynchronizedSymbols extends internal.Symbols { self: Symb
 
   trait SynchronizedSymbol extends Symbol {
 
-    override def rawflags = synchronized { super.rawflags }
-    override def rawflags_=(x: Long) = synchronized { super.rawflags_=(x) }
+    override def rawflags = gilSynchronized { super.rawflags }
+    override def rawflags_=(x: Long) = gilSynchronized { super.rawflags_=(x) }
 
-    override def rawowner = synchronized { super.rawowner }
-    override def owner_=(owner: Symbol) = synchronized { super.owner_=(owner) }
+    override def rawowner = gilSynchronized { super.rawowner }
+    override def owner_=(owner: Symbol) = gilSynchronized { super.owner_=(owner) }
 
-    override def validTo = synchronized { super.validTo }
-    override def validTo_=(x: Period) = synchronized { super.validTo_=(x) }
+    override def validTo = gilSynchronized { super.validTo }
+    override def validTo_=(x: Period) = gilSynchronized { super.validTo_=(x) }
 
-    override def pos = synchronized { super.pos }
-    override def setPos(pos: Position): this.type = { synchronized { super.setPos(pos) }; this }
+    override def pos = gilSynchronized { super.pos }
+    override def setPos(pos: Position): this.type = { gilSynchronized { super.setPos(pos) }; this }
 
-    override def privateWithin = synchronized { super.privateWithin }
-    override def privateWithin_=(sym: Symbol) = synchronized { super.privateWithin_=(sym) }
+    override def privateWithin = gilSynchronized { super.privateWithin }
+    override def privateWithin_=(sym: Symbol) = gilSynchronized { super.privateWithin_=(sym) }
 
-    override def info = synchronized { super.info }
-    override def info_=(info: Type) = synchronized { super.info_=(info) }
-    override def updateInfo(info: Type): Symbol = synchronized { super.updateInfo(info) }
-    override def rawInfo: Type = synchronized { super.rawInfo }
+    override def info = gilSynchronized { super.info }
+    override def info_=(info: Type) = gilSynchronized { super.info_=(info) }
+    override def updateInfo(info: Type): Symbol = gilSynchronized { super.updateInfo(info) }
+    override def rawInfo: Type = gilSynchronized { super.rawInfo }
 
-    override def typeParams: List[Symbol] = synchronized {
+    override def typeParams: List[Symbol] = gilSynchronized {
       if (isCompilerUniverse) super.typeParams
       else {
         if (isMonomorphicType) Nil
@@ -61,7 +65,7 @@ private[reflect] trait SynchronizedSymbols extends internal.Symbols { self: Symb
         }
       }
     }
-    override def unsafeTypeParams: List[Symbol] = synchronized {
+    override def unsafeTypeParams: List[Symbol] = gilSynchronized {
       if (isCompilerUniverse) super.unsafeTypeParams
       else {
         if (isMonomorphicType) Nil
@@ -69,12 +73,12 @@ private[reflect] trait SynchronizedSymbols extends internal.Symbols { self: Symb
       }
     }
 
-    override def reset(completer: Type): this.type = synchronized { super.reset(completer) }
+    override def reset(completer: Type): this.type = gilSynchronized { super.reset(completer) }
 
-    override def infosString: String = synchronized { super.infosString }
+    override def infosString: String = gilSynchronized { super.infosString }
 
-    override def annotations: List[AnnotationInfo] = synchronized { super.annotations }
-    override def setAnnotations(annots: List[AnnotationInfo]): this.type = { synchronized { super.setAnnotations(annots) }; this }
+    override def annotations: List[AnnotationInfo] = gilSynchronized { super.annotations }
+    override def setAnnotations(annots: List[AnnotationInfo]): this.type = { gilSynchronized { super.setAnnotations(annots) }; this }
 
 
 // ------ creators -------------------------------------------------------------------
@@ -125,40 +129,40 @@ private[reflect] trait SynchronizedSymbols extends internal.Symbols { self: Symb
 // ------- subclasses ---------------------------------------------------------------------
 
   trait SynchronizedTermSymbol extends TermSymbol with SynchronizedSymbol {
-    override def name_=(x: Name) = synchronized { super.name_=(x) }
-    override def rawname = synchronized { super.rawname }
-    override def referenced: Symbol = synchronized { super.referenced }
-    override def referenced_=(x: Symbol) = synchronized { super.referenced_=(x) }
+    override def name_=(x: Name) = gilSynchronized { super.name_=(x) }
+    override def rawname = gilSynchronized { super.rawname }
+    override def referenced: Symbol = gilSynchronized { super.referenced }
+    override def referenced_=(x: Symbol) = gilSynchronized { super.referenced_=(x) }
   }
 
   trait SynchronizedMethodSymbol extends MethodSymbol with SynchronizedTermSymbol {
-    override def typeAsMemberOf(pre: Type): Type = synchronized { super.typeAsMemberOf(pre) }
-    override def paramss: List[List[Symbol]] = synchronized { super.paramss }
-    override def returnType: Type = synchronized { super.returnType }
+    override def typeAsMemberOf(pre: Type): Type = gilSynchronized { super.typeAsMemberOf(pre) }
+    override def paramss: List[List[Symbol]] = gilSynchronized { super.paramss }
+    override def returnType: Type = gilSynchronized { super.returnType }
   }
 
   trait SynchronizedTypeSymbol extends TypeSymbol with SynchronizedSymbol {
-    override def name_=(x: Name) = synchronized { super.name_=(x) }
-    override def rawname = synchronized { super.rawname }
-    override def typeConstructor: Type = synchronized { super.typeConstructor }
-    override def tpe_* : Type = synchronized { super.tpe_* }
-    override def tpeHK : Type = synchronized { super.tpeHK }
+    override def name_=(x: Name) = gilSynchronized { super.name_=(x) }
+    override def rawname = gilSynchronized { super.rawname }
+    override def typeConstructor: Type = gilSynchronized { super.typeConstructor }
+    override def tpe_* : Type = gilSynchronized { super.tpe_* }
+    override def tpeHK : Type = gilSynchronized { super.tpeHK }
   }
 
   trait SynchronizedClassSymbol extends ClassSymbol with SynchronizedTypeSymbol {
-    override def associatedFile = synchronized { super.associatedFile }
-    override def associatedFile_=(f: AbstractFile) = synchronized { super.associatedFile_=(f) }
-    override def thisSym: Symbol = synchronized { super.thisSym }
-    override def thisType: Type = synchronized { super.thisType }
-    override def typeOfThis: Type = synchronized { super.typeOfThis }
-    override def typeOfThis_=(tp: Type) = synchronized { super.typeOfThis_=(tp) }
-    override def children = synchronized { super.children }
-    override def addChild(sym: Symbol) = synchronized { super.addChild(sym) }
+    override def associatedFile = gilSynchronized { super.associatedFile }
+    override def associatedFile_=(f: AbstractFile) = gilSynchronized { super.associatedFile_=(f) }
+    override def thisSym: Symbol = gilSynchronized { super.thisSym }
+    override def thisType: Type = gilSynchronized { super.thisType }
+    override def typeOfThis: Type = gilSynchronized { super.typeOfThis }
+    override def typeOfThis_=(tp: Type) = gilSynchronized { super.typeOfThis_=(tp) }
+    override def children = gilSynchronized { super.children }
+    override def addChild(sym: Symbol) = gilSynchronized { super.addChild(sym) }
   }
 
   trait SynchronizedModuleClassSymbol extends ModuleClassSymbol with SynchronizedClassSymbol {
-    override def sourceModule = synchronized { super.sourceModule }
-    override def implicitMembers: Scope = synchronized { super.implicitMembers }
+    override def sourceModule = gilSynchronized { super.sourceModule }
+    override def implicitMembers: Scope = gilSynchronized { super.implicitMembers }
   }
 }
 
diff --git a/src/reflect/scala/reflect/runtime/SynchronizedTypes.scala b/src/reflect/scala/reflect/runtime/SynchronizedTypes.scala
index c0146167df..4ea2d703c4 100644
--- a/src/reflect/scala/reflect/runtime/SynchronizedTypes.scala
+++ b/src/reflect/scala/reflect/runtime/SynchronizedTypes.scala
@@ -14,8 +14,9 @@ private[reflect] trait SynchronizedTypes extends internal.Types { self: SymbolTa
   // No sharing of map objects:
   override protected def commonOwnerMap = new CommonOwnerMap
 
-  private object uniqueLock
-
+  // we can keep this lock fine-grained, because super.unique just updates the cache
+  // and, in particular, doesn't call any reflection APIs which makes deadlocks impossible
+  private lazy val uniqueLock = new Object
   private val uniques = WeakHashMap[Type, WeakReference[Type]]()
   override def unique[T <: Type](tp: T): T = uniqueLock.synchronized {
     // we need to have weak uniques for runtime reflection
@@ -39,46 +40,35 @@ private[reflect] trait SynchronizedTypes extends internal.Types { self: SymbolTa
   }
 
   class SynchronizedUndoLog extends UndoLog {
-    private val actualLock = new java.util.concurrent.locks.ReentrantLock
-
-    final override def lock(): Unit = actualLock.lock()
-    final override def unlock(): Unit = actualLock.unlock()
+    final override def lock(): Unit = gil.lock()
+    final override def unlock(): Unit = gil.unlock()
   }
 
   override protected def newUndoLog = new SynchronizedUndoLog
 
   override protected def baseTypeOfNonClassTypeRef(tpe: NonClassTypeRef, clazz: Symbol) =
-    synchronized { super.baseTypeOfNonClassTypeRef(tpe, clazz) }
-
-  private object subsametypeLock
+    gilSynchronized { super.baseTypeOfNonClassTypeRef(tpe, clazz) }
 
   override def isSameType(tp1: Type, tp2: Type): Boolean =
-    subsametypeLock.synchronized { super.isSameType(tp1, tp2) }
+    gilSynchronized { super.isSameType(tp1, tp2) }
 
   override def isDifferentType(tp1: Type, tp2: Type): Boolean =
-    subsametypeLock.synchronized { super.isDifferentType(tp1, tp2) }
+    gilSynchronized { super.isDifferentType(tp1, tp2) }
 
   override def isSubType(tp1: Type, tp2: Type, depth: Depth): Boolean =
-    subsametypeLock.synchronized { super.isSubType(tp1, tp2, depth) }
-
-  private object lubglbLock
+    gilSynchronized { super.isSubType(tp1, tp2, depth) }
 
   override def glb(ts: List[Type]): Type =
-    lubglbLock.synchronized { super.glb(ts) }
+    gilSynchronized { super.glb(ts) }
 
   override def lub(ts: List[Type]): Type =
-    lubglbLock.synchronized { super.lub(ts) }
-
-  private object indentLock
-
-  override protected def explain[T](op: String, p: (Type, T) => Boolean, tp1: Type, arg2: T): Boolean = {
-    indentLock.synchronized { super.explain(op, p, tp1, arg2) }
-  }
+    gilSynchronized { super.lub(ts) }
 
-  private object toStringLock
+  override protected def explain[T](op: String, p: (Type, T) => Boolean, tp1: Type, arg2: T): Boolean =
+    gilSynchronized { super.explain(op, p, tp1, arg2) }
 
   override protected def typeToString(tpe: Type): String =
-    toStringLock.synchronized(super.typeToString(tpe))
+    gilSynchronized(super.typeToString(tpe))
 
   /* The idea of caches is as follows.
    * When in reflexive mode, a cache is either null, or one sentinal
@@ -91,18 +81,18 @@ private[reflect] trait SynchronizedTypes extends internal.Types { self: SymbolTa
    */
 
   override protected def defineUnderlyingOfSingleType(tpe: SingleType) =
-    tpe.synchronized { super.defineUnderlyingOfSingleType(tpe) }
+    gilSynchronized { super.defineUnderlyingOfSingleType(tpe) }
 
   override protected def defineBaseTypeSeqOfCompoundType(tpe: CompoundType) =
-    tpe.synchronized { super.defineBaseTypeSeqOfCompoundType(tpe) }
+    gilSynchronized { super.defineBaseTypeSeqOfCompoundType(tpe) }
 
   override protected def defineBaseClassesOfCompoundType(tpe: CompoundType) =
-    tpe.synchronized { super.defineBaseClassesOfCompoundType(tpe) }
+    gilSynchronized { super.defineBaseClassesOfCompoundType(tpe) }
 
   override protected def defineParentsOfTypeRef(tpe: TypeRef) =
-    tpe.synchronized { super.defineParentsOfTypeRef(tpe) }
+    gilSynchronized { super.defineParentsOfTypeRef(tpe) }
 
   override protected def defineBaseTypeSeqOfTypeRef(tpe: TypeRef) =
-    tpe.synchronized { super.defineBaseTypeSeqOfTypeRef(tpe) }
+    gilSynchronized { super.defineBaseTypeSeqOfTypeRef(tpe) }
 
 }
diff --git a/src/reflect/scala/reflect/runtime/TwoWayCaches.scala b/src/reflect/scala/reflect/runtime/TwoWayCaches.scala
new file mode 100644
index 0000000000..6e2890e536
--- /dev/null
+++ b/src/reflect/scala/reflect/runtime/TwoWayCaches.scala
@@ -0,0 +1,68 @@
+package scala.reflect
+package runtime
+
+import scala.collection.mutable.WeakHashMap
+import java.lang.ref.WeakReference
+
+/** A cache that maintains a bijection between Java reflection type `J`
+ *  and Scala reflection type `S`.
+ *
+ *  The cache is two-way weak (i.e. is powered by weak references),
+ *  so that neither Java artifacts prevent Scala artifacts from being garbage collected,
+ *  nor the other way around.
+ */
+private[runtime] trait TwoWayCaches { self: SymbolTable =>
+  class TwoWayCache[J, S] {
+
+    private val toScalaMap = new WeakHashMap[J, WeakReference[S]]
+    private val toJavaMap = new WeakHashMap[S, WeakReference[J]]
+
+    def enter(j: J, s: S) = gilSynchronized {
+      // debugInfo("cached: "+j+"/"+s)
+      toScalaMap(j) = new WeakReference(s)
+      toJavaMap(s) = new WeakReference(j)
+    }
+
+    private object SomeRef {
+      def unapply[T](optRef: Option[WeakReference[T]]): Option[T] =
+        if (optRef.nonEmpty) {
+          val result = optRef.get.get
+          if (result != null) Some(result) else None
+        } else None
+    }
+
+    def toScala(key: J)(body: => S): S = gilSynchronized {
+      toScalaMap get key match {
+        case SomeRef(v) =>
+          v
+        case _ =>
+          val result = body
+          enter(key, result)
+          result
+      }
+    }
+
+    def toJava(key: S)(body: => J): J = gilSynchronized {
+      toJavaMap get key match {
+        case SomeRef(v) =>
+          v
+        case _ =>
+          val result = body
+          enter(result, key)
+          result
+      }
+    }
+
+    def toJavaOption(key: S)(body: => Option[J]): Option[J] = gilSynchronized {
+      toJavaMap get key match {
+        case SomeRef(v) =>
+          Some(v)
+        case _ =>
+          val result = body
+          for (value <- result) enter(value, key)
+          result
+      }
+    }
+  }
+}
+