[hwasan] Fix missing synchronization in AllocThread.

[eugenis]

Summary

The problem was introduced in D100348.

It's really hard to trigger the bug in a stress test - the race is just too
narrow - but the new checks in Thread::Init should at least provide usable
diagnostic if the problem ever returns.

compiler-rt/lib/hwasan/hwasan_thread.cpp

void Thread::InitRandomState() {
 
   // Push a random number of zeros onto the ring buffer so that the first stack
   // tag base will be random.
   for (tag_t i = 0, e = GenerateRandomTag(); i != e; ++i)
     stack_allocations_->push(0);
 }
 
 void Thread::Init(uptr stack_buffer_start, uptr stack_buffer_size) {
+  CHECK_EQ(0, unique_id_);  // try to catch bad stack reuse
+  CHECK_EQ(0, stack_top_);
+  CHECK_EQ(0, stack_bottom_);
+
   static u64 unique_id;
   unique_id_ = unique_id++;
   if (auto sz = flags()->heap_history_size)
     heap_allocations_ = HeapAllocationsRingBuffer::New(sz);
 
   HwasanTSDThreadInit();  // Only needed with interceptors.
   uptr *ThreadLong = GetCurrentThreadLongPtr();
   // The following implicitly sets (this) as the current thread.

compiler-rt/lib/hwasan/hwasan_thread_list.h

public:
 
   ThreadStats GetThreadStats() {
     SpinMutexLock l(&stats_mutex_);
     return stats_;
   }
 
  private:
   Thread *AllocThread() {
+    SpinMutexLock l(&free_space_mutex_);
     uptr align = ring_buffer_size_ * 2;
     CHECK(IsAligned(free_space_, align));
     Thread *t = (Thread *)(free_space_ + ring_buffer_size_);
     free_space_ += thread_alloc_size_;
     CHECK(free_space_ <= free_space_end_ && "out of thread memory");
     return t;
   }
 
+  SpinMutex free_space_mutex_;
   uptr free_space_;
   uptr free_space_end_;
   uptr ring_buffer_size_;
   uptr thread_alloc_size_;
 
   SpinMutex free_list_mutex_;
   InternalMmapVector<Thread *> free_list_;
   SpinMutex live_list_mutex_;