Rewrite unwind thread handling.

This new version doesn't require any specialized thread
implementation, it uses the Current implementation to do its job.
In addition, it runs much faster when multiple threads are trying
to unwind at the same time since the global signal lock is held for
only a small amount of time. Even running through the threads one at
a time should be faster since it no longer requires two passes through
the unwound stacks.

The new code now allows multiple simultaneous unwinds of the
same thread.

Finally, add the ability to unwind from a ucontext_t passed in. This
functionality doesn't work for remote unwinds yet.

Change-Id: I4d181d7ca5ffd2acfd1686e668e6d21e36b425cb

11 files changed, 269 insertions, 258 deletions

diff --git a/include/backtrace/Backtrace.h b/include/backtrace/Backtrace.h
index 3c3a482..561dfeb 100644
--- a/include/backtrace/Backtrace.h
+++ b/include/backtrace/Backtrace.h
@@ -19,6 +19,7 @@
 
 #include <inttypes.h>
 #include <stdint.h>
+#include <ucontext.h>
 
 #include <string>
 #include <vector>
@@ -64,7 +65,7 @@ public:
   virtual ~Backtrace();
 
   // Get the current stack trace and store in the backtrace_ structure.
-  virtual bool Unwind(size_t num_ignore_frames);
+  virtual bool Unwind(size_t num_ignore_frames, ucontext_t* context = NULL);
 
   // Get the function name and offset into the function given the pc.
   // If the string is empty, then no valid function name was found.
diff --git a/libbacktrace/Android.mk b/libbacktrace/Android.mk
index a7305da..9c1b011 100755
--- a/libbacktrace/Android.mk
+++ b/libbacktrace/Android.mk
@@ -118,6 +118,9 @@ backtrace_test_shared_libraries := \
 backtrace_test_shared_libraries_target := \
 	libcutils \
 
+backtrace_test_static_libraries_host := \
+	libcutils \
+
 module := backtrace_test
 module_tag := debug
 build_type := target
diff --git a/libbacktrace/BacktraceImpl.cpp b/libbacktrace/BacktraceImpl.cpp
index 05007d9..d7ac0b2 100644
--- a/libbacktrace/BacktraceImpl.cpp
+++ b/libbacktrace/BacktraceImpl.cpp
@@ -55,8 +55,8 @@ Backtrace::~Backtrace() {
   }
 }
 
-bool Backtrace::Unwind(size_t num_ignore_frames) {
-  return impl_->Unwind(num_ignore_frames);
+bool Backtrace::Unwind(size_t num_ignore_frames, ucontext_t* ucontext) {
+  return impl_->Unwind(num_ignore_frames, ucontext);
 }
 
 extern "C" char* __cxa_demangle(const char* mangled, char* buf, size_t* len,
diff --git a/libbacktrace/BacktraceImpl.h b/libbacktrace/BacktraceImpl.h
index 7b31c38..8ed64b3 100755
--- a/libbacktrace/BacktraceImpl.h
+++ b/libbacktrace/BacktraceImpl.h
@@ -21,12 +21,13 @@
 #include <backtrace/BacktraceMap.h>
 
 #include <sys/types.h>
+#include <ucontext.h>
 
 class BacktraceImpl {
 public:
   virtual ~BacktraceImpl() { }
 
-  virtual bool Unwind(size_t num_ignore_frames) = 0;
+  virtual bool Unwind(size_t num_ignore_frames, ucontext_t* ucontext) = 0;
 
   // The name returned is not demangled, Backtrace::GetFunctionName()
   // takes care of demangling the name.
diff --git a/libbacktrace/BacktraceThread.cpp b/libbacktrace/BacktraceThread.cpp
index e0bab24..018d51f 100644
--- a/libbacktrace/BacktraceThread.cpp
+++ b/libbacktrace/BacktraceThread.cpp
@@ -16,10 +16,15 @@
 
 #include <errno.h>
 #include <inttypes.h>
+#include <limits.h>
+#include <linux/futex.h>
 #include <pthread.h>
 #include <signal.h>
 #include <string.h>
+#include <sys/syscall.h>
+#include <sys/time.h>
 #include <sys/types.h>
+#include <ucontext.h>
 
 #include <cutils/atomic.h>
 
@@ -27,190 +32,173 @@
 #include "BacktraceThread.h"
 #include "thread_utils.h"
 
+static inline int futex(volatile int* uaddr, int op, int val, const struct timespec* ts, volatile int* uaddr2, int val3) {
+  return syscall(__NR_futex, uaddr, op, val, ts, uaddr2, val3);
+}
+
 //-------------------------------------------------------------------------
 // ThreadEntry implementation.
 //-------------------------------------------------------------------------
-static ThreadEntry* g_list = NULL;
-static pthread_mutex_t g_entry_mutex = PTHREAD_MUTEX_INITIALIZER;
-static pthread_mutex_t g_sigaction_mutex = PTHREAD_MUTEX_INITIALIZER;
-
-ThreadEntry::ThreadEntry(
-    BacktraceThreadInterface* intf, pid_t pid, pid_t tid, size_t num_ignore_frames)
-    : thread_intf(intf), pid(pid), tid(tid), next(NULL), prev(NULL),
-      state(STATE_WAITING), num_ignore_frames(num_ignore_frames) {
+ThreadEntry* ThreadEntry::list_ = NULL;
+pthread_mutex_t ThreadEntry::list_mutex_ = PTHREAD_MUTEX_INITIALIZER;
+
+// Assumes that ThreadEntry::list_mutex_ has already been locked before
+// creating a ThreadEntry object.
+ThreadEntry::ThreadEntry(pid_t pid, pid_t tid)
+    : pid_(pid), tid_(tid), futex_(0), ref_count_(1), mutex_(PTHREAD_MUTEX_INITIALIZER), next_(ThreadEntry::list_), prev_(NULL) {
+  // Add ourselves to the list.
+  if (ThreadEntry::list_) {
+    ThreadEntry::list_->prev_ = this;
+  }
+  ThreadEntry::list_ = this;
 }
 
-ThreadEntry::~ThreadEntry() {
-  pthread_mutex_lock(&g_entry_mutex);
-  if (g_list == this) {
-    g_list = next;
-  } else {
-    if (next) {
-      next->prev = prev;
+ThreadEntry* ThreadEntry::Get(pid_t pid, pid_t tid, bool create) {
+  pthread_mutex_lock(&ThreadEntry::list_mutex_);
+  ThreadEntry* entry = list_;
+  while (entry != NULL) {
+    if (entry->Match(pid, tid)) {
+      break;
     }
-    prev->next = next;
+    entry = entry->next_;
   }
-  pthread_mutex_unlock(&g_entry_mutex);
 
-  next = NULL;
-  prev = NULL;
+  if (!entry) {
+    if (create) {
+      entry = new ThreadEntry(pid, tid);
+    }
+  } else {
+    entry->ref_count_++;
+  }
+  pthread_mutex_unlock(&ThreadEntry::list_mutex_);
+
+  return entry;
 }
 
-ThreadEntry* ThreadEntry::AddThreadToUnwind(
-    BacktraceThreadInterface* intf, pid_t pid, pid_t tid, size_t num_ignore_frames) {
-  ThreadEntry* entry = new ThreadEntry(intf, pid, tid, num_ignore_frames);
+void ThreadEntry::Remove(ThreadEntry* entry) {
+  pthread_mutex_unlock(&entry->mutex_);
 
-  pthread_mutex_lock(&g_entry_mutex);
-  ThreadEntry* cur_entry = g_list;
-  while (cur_entry != NULL) {
-    if (cur_entry->Match(pid, tid)) {
-      // There is already an entry for this pid/tid, this is bad.
-      BACK_LOGW("Entry for pid %d tid %d already exists.", pid, tid);
+  pthread_mutex_lock(&ThreadEntry::list_mutex_);
+  if (--entry->ref_count_ == 0) {
+    delete entry;
+  }
+  pthread_mutex_unlock(&ThreadEntry::list_mutex_);
+}
 
-      pthread_mutex_unlock(&g_entry_mutex);
-      return NULL;
+// Assumes that ThreadEntry::list_mutex_ has already been locked before
+// deleting a ThreadEntry object.
+ThreadEntry::~ThreadEntry() {
+  if (list_ == this) {
+    list_ = next_;
+  } else {
+    if (next_) {
+      next_->prev_ = prev_;
     }
-    cur_entry = cur_entry->next;
+    prev_->next_ = next_;
   }
 
-  // Add the entry to the list.
-  entry->next = g_list;
-  if (g_list) {
-    g_list->prev = entry;
+  next_ = NULL;
+  prev_ = NULL;
+}
+
+void ThreadEntry::Wait(int value) {
+  timespec ts;
+  ts.tv_sec = 10;
+  ts.tv_nsec = 0;
+  errno = 0;
+  futex(&futex_, FUTEX_WAIT, value, &ts, NULL, 0);
+  if (errno != 0 && errno != EWOULDBLOCK) {
+    BACK_LOGW("futex wait failed, futex = %d: %s", futex_, strerror(errno));
   }
-  g_list = entry;
-  pthread_mutex_unlock(&g_entry_mutex);
+}
 
-  return entry;
+void ThreadEntry::Wake() {
+  futex_++;
+  futex(&futex_, FUTEX_WAKE, INT_MAX, NULL, NULL, 0);
 }
 
 //-------------------------------------------------------------------------
 // BacktraceThread functions.
 //-------------------------------------------------------------------------
-static void SignalHandler(int n __attribute__((unused)), siginfo_t* siginfo,
-                          void* sigcontext) {
-  if (pthread_mutex_lock(&g_entry_mutex) == 0) {
-    pid_t pid = getpid();
-    pid_t tid = gettid();
-    ThreadEntry* cur_entry = g_list;
-    while (cur_entry) {
-      if (cur_entry->Match(pid, tid)) {
-        break;
-      }
-      cur_entry = cur_entry->next;
-    }
-    pthread_mutex_unlock(&g_entry_mutex);
-    if (!cur_entry) {
-      BACK_LOGW("Unable to find pid %d tid %d information", pid, tid);
-      return;
-    }
+static pthread_mutex_t g_sigaction_mutex = PTHREAD_MUTEX_INITIALIZER;
 
-    if (android_atomic_acquire_cas(STATE_WAITING, STATE_DUMPING, &cur_entry->state) == 0) {
-      cur_entry->thread_intf->ThreadUnwind(siginfo, sigcontext,
-                                           cur_entry->num_ignore_frames);
-    }
-    android_atomic_release_store(STATE_DONE, &cur_entry->state);
+static void SignalHandler(int, siginfo_t*, void* sigcontext) {
+  ThreadEntry* entry = ThreadEntry::Get(getpid(), gettid(), false);
+  if (!entry) {
+    BACK_LOGW("Unable to find pid %d tid %d information", getpid(), gettid());
+    return;
   }
+
+  entry->CopyUcontext(reinterpret_cast<ucontext_t*>(sigcontext));
+
+  // Indicate the ucontext is now valid.
+  entry->Wake();
+
+  // Pause the thread until the unwind is complete. This avoids having
+  // the thread run ahead causing problems.
+  entry->Wait(1);
+
+  ThreadEntry::Remove(entry);
 }
 
-BacktraceThread::BacktraceThread(
-    BacktraceImpl* impl, BacktraceThreadInterface* thread_intf, pid_t tid,
-    BacktraceMap* map)
-    : BacktraceCurrent(impl, map), thread_intf_(thread_intf) {
+BacktraceThread::BacktraceThread(BacktraceImpl* impl, pid_t tid, BacktraceMap* map)
+    : BacktraceCurrent(impl, map) {
   tid_ = tid;
 }
 
 BacktraceThread::~BacktraceThread() {
 }
 
-void BacktraceThread::FinishUnwind() {
-  for (std::vector<backtrace_frame_data_t>::iterator it = frames_.begin();
-       it != frames_.end(); ++it) {
-    it->map = FindMap(it->pc);
-
-    it->func_offset = 0;
-    it->func_name = GetFunctionName(it->pc, &it->func_offset);
+bool BacktraceThread::Unwind(size_t num_ignore_frames, ucontext_t* ucontext) {
+  if (ucontext) {
+    // Unwind using an already existing ucontext.
+    return impl_->Unwind(num_ignore_frames, ucontext);
   }
-}
 
-bool BacktraceThread::TriggerUnwindOnThread(ThreadEntry* entry) {
-  entry->state = STATE_WAITING;
-
-  if (tgkill(Pid(), Tid(), THREAD_SIGNAL) != 0) {
-    BACK_LOGW("tgkill failed %s", strerror(errno));
+  // Prevent multiple threads trying to set the trigger action on different
+  // threads at the same time.
+  if (pthread_mutex_lock(&g_sigaction_mutex) < 0) {
+    BACK_LOGW("sigaction failed: %s", strerror(errno));
     return false;
   }
 
-  // Allow up to ten seconds for the dump to start.
-  int wait_millis = 10000;
-  int32_t state;
-  while (true) {
-    state = android_atomic_acquire_load(&entry->state);
-    if (state != STATE_WAITING) {
-      break;
-    }
-    if (wait_millis--) {
-      usleep(1000);
-    } else {
-      break;
-    }
+  ThreadEntry* entry = ThreadEntry::Get(Pid(), Tid());
+  entry->Lock();
+
+  struct sigaction act, oldact;
+  memset(&act, 0, sizeof(act));
+  act.sa_sigaction = SignalHandler;
+  act.sa_flags = SA_RESTART | SA_SIGINFO | SA_ONSTACK;
+  sigemptyset(&act.sa_mask);
+  if (sigaction(THREAD_SIGNAL, &act, &oldact) != 0) {
+    BACK_LOGW("sigaction failed %s", strerror(errno));
+    entry->Unlock();
+    ThreadEntry::Remove(entry);
+    pthread_mutex_unlock(&g_sigaction_mutex);
+    return false;
   }
 
-  bool cancelled = false;
-  if (state == STATE_WAITING) {
-    if (android_atomic_acquire_cas(state, STATE_CANCEL, &entry->state) == 0) {
-      BACK_LOGW("Cancelled dump of thread %d", entry->tid);
-      state = STATE_CANCEL;
-      cancelled = true;
-    } else {
-      state = android_atomic_acquire_load(&entry->state);
-    }
+  if (tgkill(Pid(), Tid(), THREAD_SIGNAL) != 0) {
+    BACK_LOGW("tgkill %d failed: %s", Tid(), strerror(errno));
+    sigaction(THREAD_SIGNAL, &oldact, NULL);
+    entry->Unlock();
+    ThreadEntry::Remove(entry);
+    pthread_mutex_unlock(&g_sigaction_mutex);
+    return false;
   }
 
-  // Wait for at most ten seconds for the cancel or dump to finish.
-  wait_millis = 10000;
-  while (android_atomic_acquire_load(&entry->state) != STATE_DONE) {
-    if (wait_millis--) {
-      usleep(1000);
-    } else {
-      BACK_LOGW("Didn't finish thread unwind in 60 seconds.");
-      break;
-    }
-  }
-  return !cancelled;
-}
+  // Wait for the thread to get the ucontext.
+  entry->Wait(0);
 
-bool BacktraceThread::Unwind(size_t num_ignore_frames) {
-  ThreadEntry* entry = ThreadEntry::AddThreadToUnwind(
-      thread_intf_, Pid(), Tid(), num_ignore_frames);
-  if (!entry) {
-    return false;
-  }
+  // After the thread has received the signal, allow other unwinders to
+  // continue.
+  sigaction(THREAD_SIGNAL, &oldact, NULL);
+  pthread_mutex_unlock(&g_sigaction_mutex);
 
-  // Prevent multiple threads trying to set the trigger action on different
-  // threads at the same time.
-  bool retval = false;
-  if (pthread_mutex_lock(&g_sigaction_mutex) == 0) {
-    struct sigaction act, oldact;
-    memset(&act, 0, sizeof(act));
-    act.sa_sigaction = SignalHandler;
-    act.sa_flags = SA_RESTART | SA_SIGINFO | SA_ONSTACK;
-    sigemptyset(&act.sa_mask);
-    if (sigaction(THREAD_SIGNAL, &act, &oldact) == 0) {
-      retval = TriggerUnwindOnThread(entry);
-      sigaction(THREAD_SIGNAL, &oldact, NULL);
-    } else {
-      BACK_LOGW("sigaction failed %s", strerror(errno));
-    }
-    pthread_mutex_unlock(&g_sigaction_mutex);
-  } else {
-    BACK_LOGW("unable to acquire sigaction mutex.");
-  }
+  bool unwind_done = impl_->Unwind(num_ignore_frames, entry->GetUcontext());
 
-  if (retval) {
-    FinishUnwind();
-  }
-  delete entry;
+  // Tell the signal handler to exit and release the entry.
+  entry->Wake();
 
-  return retval;
+  return unwind_done;
 }
diff --git a/libbacktrace/BacktraceThread.h b/libbacktrace/BacktraceThread.h
index 9310a44..a75a807 100644
--- a/libbacktrace/BacktraceThread.h
+++ b/libbacktrace/BacktraceThread.h
@@ -18,18 +18,14 @@
 #define _LIBBACKTRACE_BACKTRACE_THREAD_H
 
 #include <inttypes.h>
+#include <pthread.h>
 #include <signal.h>
+#include <string.h>
 #include <sys/types.h>
+#include <ucontext.h>
 
 #include "BacktraceImpl.h"
 
-enum state_e {
-  STATE_WAITING = 0,
-  STATE_DUMPING,
-  STATE_DONE,
-  STATE_CANCEL,
-};
-
 // The signal used to cause a thread to dump the stack.
 #if defined(__GLIBC__)
 // GLIBC reserves __SIGRTMIN signals, so use SIGRTMIN to avoid errors.
@@ -38,62 +34,57 @@ enum state_e {
 #define THREAD_SIGNAL (__SIGRTMIN+1)
 #endif
 
-class BacktraceThreadInterface;
+class ThreadEntry {
+public:
+  static ThreadEntry* Get(pid_t pid, pid_t tid, bool create = true);
 
-struct ThreadEntry {
-  ThreadEntry(
-      BacktraceThreadInterface* impl, pid_t pid, pid_t tid,
-      size_t num_ignore_frames);
-  ~ThreadEntry();
+  static void Remove(ThreadEntry* entry);
 
-  bool Match(pid_t chk_pid, pid_t chk_tid) { return (chk_pid == pid && chk_tid == tid); }
+  inline void CopyUcontext(ucontext_t* ucontext) {
+    memcpy(&ucontext_, ucontext, sizeof(ucontext_));
+  }
 
-  static ThreadEntry* AddThreadToUnwind(
-      BacktraceThreadInterface* thread_intf, pid_t pid, pid_t tid,
-      size_t num_ignored_frames);
+  void Wake();
 
-  BacktraceThreadInterface* thread_intf;
-  pid_t pid;
-  pid_t tid;
-  ThreadEntry* next;
-  ThreadEntry* prev;
-  int32_t state;
-  int num_ignore_frames;
-};
+  void Wait(int);
 
-// Interface class that does not contain any local storage, only defines
-// virtual functions to be defined by subclasses.
-class BacktraceThreadInterface {
-public:
-  virtual ~BacktraceThreadInterface() { }
+  inline void Lock() {
+    pthread_mutex_lock(&mutex_);
+    // Reset the futex value in case of multiple unwinds of the same thread.
+    futex_ = 0;
+  }
 
-  virtual void ThreadUnwind(
-      siginfo_t* siginfo, void* sigcontext, size_t num_ignore_frames) = 0;
-};
+  inline void Unlock() {
+    pthread_mutex_unlock(&mutex_);
+  }
 
-class BacktraceThread : public BacktraceCurrent {
-public:
-  // impl and thread_intf should point to the same object, this allows
-  // the compiler to catch if an implementation does not properly
-  // subclass both.
-  BacktraceThread(
-      BacktraceImpl* impl, BacktraceThreadInterface* thread_intf, pid_t tid,
-      BacktraceMap* map);
-  virtual ~BacktraceThread();
+  inline ucontext_t* GetUcontext() { return &ucontext_; }
 
-  virtual bool Unwind(size_t num_ignore_frames);
+private:
+  ThreadEntry(pid_t pid, pid_t tid);
+  ~ThreadEntry();
 
-  virtual void ThreadUnwind(
-      siginfo_t* siginfo, void* sigcontext, size_t num_ignore_frames) {
-    thread_intf_->ThreadUnwind(siginfo, sigcontext, num_ignore_frames);
-  }
+  bool Match(pid_t chk_pid, pid_t chk_tid) { return (chk_pid == pid_ && chk_tid == tid_); }
 
-private:
-  virtual bool TriggerUnwindOnThread(ThreadEntry* entry);
+  pid_t pid_;
+  pid_t tid_;
+  int futex_;
+  int ref_count_;
+  pthread_mutex_t mutex_;
+  ThreadEntry* next_;
+  ThreadEntry* prev_;
+  ucontext_t ucontext_;
 
-  virtual void FinishUnwind();
+  static ThreadEntry* list_;
+  static pthread_mutex_t list_mutex_;
+};
+
+class BacktraceThread : public BacktraceCurrent {
+public:
+  BacktraceThread(BacktraceImpl* impl, pid_t tid, BacktraceMap* map);
+  virtual ~BacktraceThread();
 
-  BacktraceThreadInterface* thread_intf_;
+  virtual bool Unwind(size_t num_ignore_frames, ucontext_t* ucontext);
 };
 
 #endif // _LIBBACKTRACE_BACKTRACE_THREAD_H
diff --git a/libbacktrace/UnwindCurrent.cpp b/libbacktrace/UnwindCurrent.cpp
index 67d372a..e5f7c3d 100755
--- a/libbacktrace/UnwindCurrent.cpp
+++ b/libbacktrace/UnwindCurrent.cpp
@@ -24,6 +24,7 @@
 #include <libunwind.h>
 
 #include "BacktraceLog.h"
+#include "BacktraceThread.h"
 #include "UnwindCurrent.h"
 #include "UnwindMap.h"
 
@@ -36,15 +37,45 @@ UnwindCurrent::UnwindCurrent() {
 UnwindCurrent::~UnwindCurrent() {
 }
 
-bool UnwindCurrent::Unwind(size_t num_ignore_frames) {
-  int ret = unw_getcontext(&context_);
-  if (ret < 0) {
-    BACK_LOGW("unw_getcontext failed %d", ret);
-    return false;
+bool UnwindCurrent::Unwind(size_t num_ignore_frames, ucontext_t* ucontext) {
+  if (!ucontext) {
+    int ret = unw_getcontext(&context_);
+    if (ret < 0) {
+      BACK_LOGW("unw_getcontext failed %d", ret);
+      return false;
+    }
+  }
+  else {
+    GetUnwContextFromUcontext(ucontext);
   }
   return UnwindFromContext(num_ignore_frames, false);
 }
 
+void UnwindCurrent::GetUnwContextFromUcontext(const ucontext_t* ucontext) {
+  unw_tdep_context_t* unw_context = reinterpret_cast<unw_tdep_context_t*>(&context_);
+
+#if defined(__arm__)
+  unw_context->regs[0] = ucontext->uc_mcontext.arm_r0;
+  unw_context->regs[1] = ucontext->uc_mcontext.arm_r1;
+  unw_context->regs[2] = ucontext->uc_mcontext.arm_r2;
+  unw_context->regs[3] = ucontext->uc_mcontext.arm_r3;
+  unw_context->regs[4] = ucontext->uc_mcontext.arm_r4;
+  unw_context->regs[5] = ucontext->uc_mcontext.arm_r5;
+  unw_context->regs[6] = ucontext->uc_mcontext.arm_r6;
+  unw_context->regs[7] = ucontext->uc_mcontext.arm_r7;
+  unw_context->regs[8] = ucontext->uc_mcontext.arm_r8;
+  unw_context->regs[9] = ucontext->uc_mcontext.arm_r9;
+  unw_context->regs[10] = ucontext->uc_mcontext.arm_r10;
+  unw_context->regs[11] = ucontext->uc_mcontext.arm_fp;
+  unw_context->regs[12] = ucontext->uc_mcontext.arm_ip;
+  unw_context->regs[13] = ucontext->uc_mcontext.arm_sp;
+  unw_context->regs[14] = ucontext->uc_mcontext.arm_lr;
+  unw_context->regs[15] = ucontext->uc_mcontext.arm_pc;
+#else
+  unw_context->uc_mcontext = ucontext->uc_mcontext;
+#endif
+}
+
 std::string UnwindCurrent::GetFunctionNameRaw(uintptr_t pc, uintptr_t* offset) {
   *offset = 0;
   char buf[512];
@@ -122,47 +153,6 @@ bool UnwindCurrent::UnwindFromContext(size_t num_ignore_frames, bool within_hand
   return true;
 }
 
-void UnwindCurrent::ExtractContext(void* sigcontext) {
-  unw_tdep_context_t* context = reinterpret_cast<unw_tdep_context_t*>(&context_);
-  const ucontext_t* uc = reinterpret_cast<const ucontext_t*>(sigcontext);
-
-#if defined(__arm__)
-  context->regs[0] = uc->uc_mcontext.arm_r0;
-  context->regs[1] = uc->uc_mcontext.arm_r1;
-  context->regs[2] = uc->uc_mcontext.arm_r2;
-  context->regs[3] = uc->uc_mcontext.arm_r3;
-  context->regs[4] = uc->uc_mcontext.arm_r4;
-  context->regs[5] = uc->uc_mcontext.arm_r5;
-  context->regs[6] = uc->uc_mcontext.arm_r6;
-  context->regs[7] = uc->uc_mcontext.arm_r7;
-  context->regs[8] = uc->uc_mcontext.arm_r8;
-  context->regs[9] = uc->uc_mcontext.arm_r9;
-  context->regs[10] = uc->uc_mcontext.arm_r10;
-  context->regs[11] = uc->uc_mcontext.arm_fp;
-  context->regs[12] = uc->uc_mcontext.arm_ip;
-  context->regs[13] = uc->uc_mcontext.arm_sp;
-  context->regs[14] = uc->uc_mcontext.arm_lr;
-  context->regs[15] = uc->uc_mcontext.arm_pc;
-#else
-  context->uc_mcontext = uc->uc_mcontext;
-#endif
-}
-
-//-------------------------------------------------------------------------
-// UnwindThread functions.
-//-------------------------------------------------------------------------
-UnwindThread::UnwindThread() {
-}
-
-UnwindThread::~UnwindThread() {
-}
-
-void UnwindThread::ThreadUnwind(
-    siginfo_t* /*siginfo*/, void* sigcontext, size_t num_ignore_frames) {
-  ExtractContext(sigcontext);
-  UnwindFromContext(num_ignore_frames, true);
-}
-
 //-------------------------------------------------------------------------
 // C++ object creation function.
 //-------------------------------------------------------------------------
@@ -171,6 +161,5 @@ Backtrace* CreateCurrentObj(BacktraceMap* map) {
 }
 
 Backtrace* CreateThreadObj(pid_t tid, BacktraceMap* map) {
-  UnwindThread* thread_obj = new UnwindThread();
-  return new BacktraceThread(thread_obj, thread_obj, tid, map);
+  return new BacktraceThread(new UnwindCurrent(), tid, map);
 }
diff --git a/libbacktrace/UnwindCurrent.h b/libbacktrace/UnwindCurrent.h
index 41080c7..2375e6e 100644
--- a/libbacktrace/UnwindCurrent.h
+++ b/libbacktrace/UnwindCurrent.h
@@ -20,7 +20,6 @@
 #include <string>
 
 #include "BacktraceImpl.h"
-#include "BacktraceThread.h"
 
 #define UNW_LOCAL_ONLY
 #include <libunwind.h>
@@ -30,25 +29,16 @@ public:
   UnwindCurrent();
   virtual ~UnwindCurrent();
 
-  virtual bool Unwind(size_t num_ignore_frames);
+  virtual bool Unwind(size_t num_ignore_frames, ucontext_t* ucontext);
 
   virtual std::string GetFunctionNameRaw(uintptr_t pc, uintptr_t* offset);
 
   bool UnwindFromContext(size_t num_ignore_frames, bool within_handler);
 
-  void ExtractContext(void* sigcontext);
+  void GetUnwContextFromUcontext(const ucontext_t* context);
 
 protected:
   unw_context_t context_;
 };
 
-class UnwindThread : public UnwindCurrent, public BacktraceThreadInterface {
-public:
-  UnwindThread();
-  virtual ~UnwindThread();
-
-  virtual void ThreadUnwind(
-      siginfo_t* siginfo, void* sigcontext, size_t num_ignore_frames);
-};
-
 #endif // _LIBBACKTRACE_UNWIND_CURRENT_H
diff --git a/libbacktrace/UnwindPtrace.cpp b/libbacktrace/UnwindPtrace.cpp
index 5ca7e60..63b4f16 100644
--- a/libbacktrace/UnwindPtrace.cpp
+++ b/libbacktrace/UnwindPtrace.cpp
@@ -45,7 +45,12 @@ UnwindPtrace::~UnwindPtrace() {
   }
 }
 
-bool UnwindPtrace::Unwind(size_t num_ignore_frames) {
+bool UnwindPtrace::Unwind(size_t num_ignore_frames, ucontext_t* ucontext) {
+  if (ucontext) {
+    BACK_LOGW("Unwinding from a specified context not supported yet.");
+    return false;
+  }
+
   addr_space_ = unw_create_addr_space(&_UPT_accessors, 0);
   if (!addr_space_) {
     BACK_LOGW("unw_create_addr_space failed.");
diff --git a/libbacktrace/UnwindPtrace.h b/libbacktrace/UnwindPtrace.h
index 1e82117..2fb7967 100644
--- a/libbacktrace/UnwindPtrace.h
+++ b/libbacktrace/UnwindPtrace.h
@@ -28,7 +28,7 @@ public:
   UnwindPtrace();
   virtual ~UnwindPtrace();
 
-  virtual bool Unwind(size_t num_ignore_frames);
+  virtual bool Unwind(size_t num_ignore_frames, ucontext_t* ucontext);
 
   virtual std::string GetFunctionNameRaw(uintptr_t pc, uintptr_t* offset);
 
diff --git a/libbacktrace/backtrace_test.cpp b/libbacktrace/backtrace_test.cpp
index 9744922..ed6b211 100644
--- a/libbacktrace/backtrace_test.cpp
+++ b/libbacktrace/backtrace_test.cpp
@@ -615,6 +615,49 @@ TEST(libbacktrace, thread_multiple_dump) {
   }
 }
 
+TEST(libbacktrace, thread_multiple_dump_same_thread) {
+  pthread_attr_t attr;
+  pthread_attr_init(&attr);
+  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
+  thread_t runner;
+  runner.tid = 0;
+  runner.state = 0;
+  ASSERT_TRUE(pthread_create(&runner.threadId, &attr, ThreadMaxRun, &runner) == 0);
+
+  // Wait for tids to be set.
+  ASSERT_TRUE(WaitForNonZero(&runner.state, 10));
+
+  // Start all of the dumpers at once, they will spin until they are signalled
+  // to begin their dump run.
+  int32_t dump_now = 0;
+  // Dump the same thread NUM_THREADS simultaneously.
+  std::vector<dump_thread_t> dumpers(NUM_THREADS);
+  for (size_t i = 0; i < NUM_THREADS; i++) {
+    dumpers[i].thread.tid = runner.tid;
+    dumpers[i].thread.state = 0;
+    dumpers[i].done = 0;
+    dumpers[i].now = &dump_now;
+
+    ASSERT_TRUE(pthread_create(&dumpers[i].thread.threadId, &attr, ThreadDump, &dumpers[i]) == 0);
+  }
+
+  // Start all of the dumpers going at once.
+  android_atomic_acquire_store(1, &dump_now);
+
+  for (size_t i = 0; i < NUM_THREADS; i++) {
+    ASSERT_TRUE(WaitForNonZero(&dumpers[i].done, 100));
+
+    ASSERT_TRUE(dumpers[i].backtrace != NULL);
+    VerifyMaxDump(dumpers[i].backtrace);
+
+    delete dumpers[i].backtrace;
+    dumpers[i].backtrace = NULL;
+  }
+
+  // Tell the runner thread to exit its infinite loop.
+  android_atomic_acquire_store(0, &runner.state);
+}
+
 // This test is for UnwindMaps that should share the same map cursor when
 // multiple maps are created for the current process at the same time.
 TEST(libbacktrace, simultaneous_maps) {