[vm] If waiting for an active mutator slot for too long, steal a slot from a thread at a native safepoint.

The thread whose slot was stolen will take the slow path attempting to exit its safepoint and wait to reacquire an active mutator slot.

TEST=ci
Bug: #54687
Change-Id: I49c4c12b38d38d7afa4df6b346c2a464f75ef6e0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/406404
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: rmacnak-google

runtime/bin/dartutils.h

@@ -651,20 +651,6 @@ class ScopedBlockingCall {
   DISALLOW_COPY_AND_ASSIGN(ScopedBlockingCall);
 };
 
-// Remove once we remove the limitation on the number of running mutators.
-// https://github.com/dart-lang/sdk/issues/54687
-class LeaveIsolateScope {
- public:
-  LeaveIsolateScope() : isolate_(Dart_CurrentIsolate()) { Dart_ExitIsolate(); }
-  ~LeaveIsolateScope() { Dart_EnterIsolate(isolate_); }
-
- private:
-  Dart_Isolate isolate_;
-
-  DISALLOW_ALLOCATION();
-  DISALLOW_COPY_AND_ASSIGN(LeaveIsolateScope);
-};
-
 struct MagicNumberData {
   static constexpr intptr_t kMaxLength = 8;
 

runtime/bin/process.cc

@@ -285,8 +285,6 @@ void FUNCTION_NAME(Process_Sleep)(Dart_NativeArguments args) {
   int64_t milliseconds = 0;
   // Ignore result if passing invalid argument and just set exit code to 0.
   DartUtils::GetInt64Value(Dart_GetNativeArgument(args, 0), &milliseconds);
-
-  LeaveIsolateScope leave_isolate;
   TimerUtils::Sleep(milliseconds);
 }
 

runtime/bin/process_linux.cc

@@ -861,8 +861,6 @@ bool Process::Wait(intptr_t pid,
 
   int alive = 3;
   while (alive > 0) {
-    LeaveIsolateScope leave_isolate;
-
     // Blocking call waiting for events from the child process.
     if (TEMP_FAILURE_RETRY(poll(fds, alive, -1)) <= 0) {
       return CloseProcessBuffers(fds, alive);

runtime/bin/process_macos.cc

@@ -826,8 +826,6 @@ bool Process::Wait(intptr_t pid,
 
   int alive = 3;
   while (alive > 0) {
-    LeaveIsolateScope leave_isolate;
-
     // Blocking call waiting for events from the child process.
     if (TEMP_FAILURE_RETRY(poll(fds, alive, -1)) <= 0) {
       return CloseProcessBuffers(fds, alive);

runtime/bin/process_win.cc

@@ -866,8 +866,6 @@ bool Process::Wait(intptr_t pid,
   // Continue until all handles are closed.
   int alive = kHandles;
   while (alive > 0) {
-    LeaveIsolateScope leave_isolate;
-
     // Blocking call waiting for events from the child process.
     DWORD wait_result = WaitForMultipleObjects(alive, events, FALSE, INFINITE);
 

runtime/tests/vm/dart/isolates/many_isolates_blocked_at_ffi_test.dart

@@ -0,0 +1,33 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "dart:isolate";
+import "dart:io";
+import "dart:ffi";
+
+typedef CSleep = Void Function(Int32);
+typedef DartSleep = void Function(int);
+
+child(replyPort) {
+  replyPort.send(null);
+
+  var sleep = DynamicLibrary.process().lookupFunction<CSleep, DartSleep>(
+    "sleep",
+  );
+  sleep(60 * 60);
+}
+
+main() async {
+  var pending = 0;
+  var port;
+  port = new RawReceivePort((msg) {
+    pending--;
+    if (pending == 0) exit(0);
+  });
+
+  for (var i = 0; i < 20; i++) {
+    Isolate.spawn(child, port.sendPort);
+    pending++;
+  }
+}

runtime/tests/vm/vm.status

@@ -56,6 +56,9 @@ dart/kernel_determinism_test: SkipSlow
 dart/minimal_kernel_test: SkipSlow # gen_kernel is too slow with optimization_counter_threshold
 
 [ $compiler == app_jitk ]
+dart/isolates/many_isolates_blocked_at_ffi_test: Skip # App snapshot creation needs regular isolate shutdown
+dart/isolates/many_isolates_blocked_at_process_run_sync_test: Skip # App snapshot creation needs regular isolate shutdown
+dart/isolates/many_isolates_blocked_at_sleep_test: Skip # App snapshot creation needs regular isolate shutdown
 dart/snapshot_version_test: RuntimeError
 
 [ $compiler == dartk ]

runtime/vm/compiler/assembler/assembler_arm.cc

@@ -590,10 +590,10 @@ void Assembler::EnterFullSafepoint(Register addr, Register state) {
   add(addr, THR, Operand(addr));
   Bind(&retry);
   ldrex(state, addr);
-  cmp(state, Operand(target::Thread::full_safepoint_state_unacquired()));
+  cmp(state, Operand(target::Thread::native_safepoint_state_unacquired()));
   b(&slow_path, NE);
 
-  mov(state, Operand(target::Thread::full_safepoint_state_acquired()));
+  mov(state, Operand(target::Thread::native_safepoint_state_acquired()));
   strex(TMP, state, addr);
   cmp(TMP, Operand(0));  // 0 means strex was successful.
   b(&done, EQ);
@@ -651,10 +651,10 @@ void Assembler::ExitFullSafepoint(Register tmp1,
   add(addr, THR, Operand(addr));
   Bind(&retry);
   ldrex(state, addr);
-  cmp(state, Operand(target::Thread::full_safepoint_state_acquired()));
+  cmp(state, Operand(target::Thread::native_safepoint_state_acquired()));
   b(&slow_path, NE);
 
-  mov(state, Operand(target::Thread::full_safepoint_state_unacquired()));
+  mov(state, Operand(target::Thread::native_safepoint_state_unacquired()));
   strex(TMP, state, addr);
   cmp(TMP, Operand(0));  // 0 means strex was successful.
   b(&done, EQ);
@@ -690,8 +690,8 @@ void Assembler::TransitionNativeToGenerated(Register addr,
     ASSERT(!ignore_unwind_in_progress);
 #if defined(DEBUG)
     // Ensure we've already left the safepoint.
-    ASSERT(target::Thread::full_safepoint_state_acquired() != 0);
-    LoadImmediate(state, target::Thread::full_safepoint_state_acquired());
+    ASSERT(target::Thread::native_safepoint_state_acquired() != 0);
+    LoadImmediate(state, target::Thread::native_safepoint_state_acquired());
     ldr(TMP, Address(THR, target::Thread::safepoint_state_offset()));
     ands(TMP, TMP, Operand(state));
     Label ok;

runtime/vm/compiler/assembler/assembler_arm64.cc

@@ -1564,10 +1564,10 @@ void Assembler::EnterFullSafepoint(Register state) {
   add(addr, THR, Operand(addr));
   Bind(&retry);
   ldxr(state, addr);
-  cmp(state, Operand(target::Thread::full_safepoint_state_unacquired()));
+  cmp(state, Operand(target::Thread::native_safepoint_state_unacquired()));
   b(&slow_path, NE);
 
-  movz(state, Immediate(target::Thread::full_safepoint_state_acquired()), 0);
+  movz(state, Immediate(target::Thread::native_safepoint_state_acquired()), 0);
   stxr(TMP, state, addr);
   cbz(&done, TMP);  // 0 means stxr was successful.
 
@@ -1637,10 +1637,11 @@ void Assembler::ExitFullSafepoint(Register state,
   add(addr, THR, Operand(addr));
   Bind(&retry);
   ldxr(state, addr);
-  cmp(state, Operand(target::Thread::full_safepoint_state_acquired()));
+  cmp(state, Operand(target::Thread::native_safepoint_state_acquired()));
   b(&slow_path, NE);
 
-  movz(state, Immediate(target::Thread::full_safepoint_state_unacquired()), 0);
+  movz(state, Immediate(target::Thread::native_safepoint_state_unacquired()),
+       0);
   stxr(TMP, state, addr);
   cbz(&done, TMP);  // 0 means stxr was successful.
 
@@ -1674,8 +1675,8 @@ void Assembler::TransitionNativeToGenerated(Register state,
     ASSERT(!ignore_unwind_in_progress);
 #if defined(DEBUG)
     // Ensure we've already left the safepoint.
-    ASSERT(target::Thread::full_safepoint_state_acquired() != 0);
-    LoadImmediate(state, target::Thread::full_safepoint_state_acquired());
+    ASSERT(target::Thread::native_safepoint_state_acquired() != 0);
+    LoadImmediate(state, target::Thread::native_safepoint_state_acquired());
     ldr(TMP, Address(THR, target::Thread::safepoint_state_offset()));
     and_(TMP, TMP, Operand(state));
     Label ok;

runtime/vm/compiler/assembler/assembler_ia32.cc

@@ -2495,12 +2495,12 @@ void Assembler::EnterFullSafepoint(Register scratch) {
   }
 
   pushl(EAX);
-  movl(EAX, Immediate(target::Thread::full_safepoint_state_unacquired()));
-  movl(scratch, Immediate(target::Thread::full_safepoint_state_acquired()));
+  movl(EAX, Immediate(target::Thread::native_safepoint_state_unacquired()));
+  movl(scratch, Immediate(target::Thread::native_safepoint_state_acquired()));
   LockCmpxchgl(Address(THR, target::Thread::safepoint_state_offset()), scratch);
   movl(scratch, EAX);
   popl(EAX);
-  cmpl(scratch, Immediate(target::Thread::full_safepoint_state_unacquired()));
+  cmpl(scratch, Immediate(target::Thread::native_safepoint_state_unacquired()));
 
   if (!FLAG_use_slow_path) {
     j(EQUAL, &done);
@@ -2551,12 +2551,12 @@ void Assembler::ExitFullSafepoint(Register scratch,
   }
 
   pushl(EAX);
-  movl(EAX, Immediate(target::Thread::full_safepoint_state_acquired()));
-  movl(scratch, Immediate(target::Thread::full_safepoint_state_unacquired()));
+  movl(EAX, Immediate(target::Thread::native_safepoint_state_acquired()));
+  movl(scratch, Immediate(target::Thread::native_safepoint_state_unacquired()));
   LockCmpxchgl(Address(THR, target::Thread::safepoint_state_offset()), scratch);
   movl(scratch, EAX);
   popl(EAX);
-  cmpl(scratch, Immediate(target::Thread::full_safepoint_state_acquired()));
+  cmpl(scratch, Immediate(target::Thread::native_safepoint_state_acquired()));
 
   if (!FLAG_use_slow_path) {
     j(EQUAL, &done);
@@ -2589,7 +2589,7 @@ void Assembler::TransitionNativeToGenerated(Register scratch,
 #if defined(DEBUG)
     // Ensure we've already left the safepoint.
     movl(scratch, Address(THR, target::Thread::safepoint_state_offset()));
-    andl(scratch, Immediate(target::Thread::full_safepoint_state_acquired()));
+    andl(scratch, Immediate(target::Thread::native_safepoint_state_acquired()));
     Label ok;
     j(ZERO, &ok);
     Breakpoint();