[vm] Drop Android 6.0.0 specific workaround

Flutter does not support Android versions below Android 7 (API level 
24).

Simplify the code by removing workaround introduced in 
a013de84e0 to work-around crashes
which only happen on Android 6.0.0.

TEST=ci

Cq-Include-Trybots: luci.dart.try:vm-aot-android-release-arm_x64-try,vm-aot-android-release-arm64c-try
Change-Id: Ib29535e66e87b17bc9b2ac72bcbb46e06a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/472861
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>

runtime/vm/os_thread.cc

@@ -115,13 +115,6 @@ OSThread::~OSThread() {
 #endif
   timeline_block_ = nullptr;
   free(name_);
-#if !defined(PRODUCT)
-  if (prepared_for_interrupts_) {
-    ThreadInterrupter::CleanupCurrentThreadState(thread_interrupter_state_);
-    thread_interrupter_state_ = nullptr;
-    prepared_for_interrupts_ = false;
-  }
-#endif  // !defined(PRODUCT)
 }
 
 void OSThread::SetName(const char* name) {
@@ -161,12 +154,6 @@ void OSThread::EnableThreadInterrupts() {
   ASSERT(OSThread::Current() == this);
   uintptr_t old = thread_interrupt_disabled_.fetch_sub(1u);
   if (FLAG_profiler && (old == 1)) {
-    // We just decremented from 1 to 0.
-    // Make sure the thread interrupter is awake.
-    if (!prepared_for_interrupts_) {
-      thread_interrupter_state_ = ThreadInterrupter::PrepareCurrentThread();
-      prepared_for_interrupts_ = true;
-    }
     ThreadInterrupter::WakeUp();
   }
   if (old == 0) {

runtime/vm/os_thread.h

@@ -297,8 +297,6 @@ class OSThread : public BaseThread {
 #if defined(DART_INCLUDE_PROFILER)
   // Thread interrupts disabled by default.
   RelaxedAtomic<uintptr_t> thread_interrupt_disabled_ = {1};
-  bool prepared_for_interrupts_ = false;
-  void* thread_interrupter_state_ = nullptr;
 #endif  // defined(DART_INCLUDE_PROFILER)
 
   Log* log_;

runtime/vm/signal_handler.h

@@ -51,15 +51,6 @@ class SignalHandler : public AllStatic {
   static uintptr_t GetCStackPointer(const mcontext_t& mcontext);
   static uintptr_t GetDartStackPointer(const mcontext_t& mcontext);
   static uintptr_t GetLinkRegister(const mcontext_t& mcontext);
-
-#if defined(DART_HOST_OS_ANDROID)
-  // Prepare current thread for handling interrupts. Returns
-  // opaque pointer to the allocated state (if any).
-  static void* PrepareCurrentThread();
-
-  // Cleanup any state which was created by |PrepareCurrentThread|.
-  static void CleanupCurrentThreadState(void* state);
-#endif
 };
 
 #undef USE_SIGNAL_HANDLER_TRAMPOLINE

runtime/vm/signal_handler_android.cc

@@ -136,55 +136,6 @@ void SignalHandler::Remove() {
   RELEASE_ASSERT(r == 0);
 }
 
-void* SignalHandler::PrepareCurrentThread() {
-  // These constants are selected to prevent allocating alternative signal
-  // stack if Bionic has already allocated large enough one for us. They
-  // match current values used in Bionic[1].
-  //
-  // [1]: https://cs.android.com/android/platform/superproject/main/+/main:bionic/libc/bionic/pthread_internal.h;drc=3649db34a154cedb8ef53a5adbaa349970159b58;l=243
-  const intptr_t kGuardPageSize = 4 * KB;
-#if defined(TARGET_ARCH_IS_64_BIT)
-  const intptr_t kSigAltStackSize = 32 * KB;
-#else
-  const intptr_t kSigAltStackSize = 16 * KB;
-#endif
-
-  // First check if the alternative signal stack is already installed and
-  // large enough.
-  int r;
-  stack_t ss;
-  memset(&ss, 0, sizeof(ss));
-  r = sigaltstack(nullptr, &ss);
-  ASSERT(r == 0);
-  if (ss.ss_flags == 0 && ss.ss_size >= (kSigAltStackSize - kGuardPageSize)) {
-    // Bionic has created a large enough stack already.
-    return nullptr;
-  }
-
-  // We are running on an older version of Android, where Bionic creates
-  // stacks which are too small.
-  ss.ss_sp = malloc(kSigAltStackSize);
-  ss.ss_size = kSigAltStackSize;
-  ss.ss_flags = 0;
-  r = sigaltstack(&ss, nullptr);
-  ASSERT(r == 0);
-
-  return ss.ss_sp;
-}
-
-void SignalHandler::CleanupCurrentThreadState(void* stack) {
-  if (stack != nullptr) {
-    // Disable alternative stack then free allocated memory.
-    stack_t ss, old_ss;
-    memset(&ss, 0, sizeof(ss));
-    ss.ss_flags = SS_DISABLE;
-    int r = sigaltstack(&ss, &old_ss);
-    ASSERT(r == 0);
-    ASSERT(old_ss.ss_sp == stack);
-    free(stack);
-  }
-}
-
 }  // namespace dart
 
 #endif  // defined(DART_HOST_OS_ANDROID)

runtime/vm/thread_interrupter.cc

@@ -231,14 +231,6 @@ void ThreadInterrupter::ThreadMain(uword parameters) {
   }
 }
 
-#if !defined(DART_HOST_OS_ANDROID)
-void* ThreadInterrupter::PrepareCurrentThread() {
-  return nullptr;
-}
-
-void ThreadInterrupter::CleanupCurrentThreadState(void* state) {}
-#endif
-
 #endif  // defined(DART_INCLUDE_PROFILER)
 
 }  // namespace dart

runtime/vm/thread_interrupter.h

@@ -39,13 +39,6 @@ class ThreadInterrupter : public AllStatic {
   // Interrupt a thread.
   static void InterruptThread(OSThread* thread);
 
-  // Prepare current thread for handling interrupts. Returns
-  // opaque pointer to the allocated state (if any).
-  static void* PrepareCurrentThread();
-
-  // Cleanup any state which was created by |PrepareCurrentThread|.
-  static void CleanupCurrentThreadState(void* state);
-
  private:
   static constexpr intptr_t kMaxThreads = 4096;
   static bool initialized_;

runtime/vm/thread_interrupter_android.cc

@@ -147,14 +147,6 @@ void ThreadInterrupter::RemoveSignalHandler() {
   SignalHandler::Remove();
 }
 
-void* ThreadInterrupter::PrepareCurrentThread() {
-  return SignalHandler::PrepareCurrentThread();
-}
-
-void ThreadInterrupter::CleanupCurrentThreadState(void* state) {
-  SignalHandler::CleanupCurrentThreadState(state);
-}
-
 #endif  // defined(DART_INCLUDE_PROFILER)
 
 }  // namespace dart