tsan: fix trace initialization during thread id reuse

The current code leaves the first event in the trace part uninitialized
(from the previous thread). It can cause unpredictable behavior
during stack/mutexset restoration.
Initialize the first event to a fake harmless memory access.

llvm-svn: 224834

1 files changed, 5 insertions, 7 deletions

diff --git a/compiler-rt/lib/tsan/rtl/tsan_rtl_thread.cc b/compiler-rt/lib/tsan/rtl/tsan_rtl_thread.cc
index d84cd44b554..065ba88439c 100644
--- a/compiler-rt/lib/tsan/rtl/tsan_rtl_thread.cc
+++ b/compiler-rt/lib/tsan/rtl/tsan_rtl_thread.cc
@@ -111,15 +111,13 @@ void ThreadContext::OnStarted(void *arg) {
     thr->dd_pt = ctx->dd->CreatePhysicalThread();
     thr->dd_lt = ctx->dd->CreateLogicalThread(unique_id);
   }
+  thr->fast_state.SetHistorySize(flags()->history_size);
+  // Commit switch to the new part of the trace.
+  // TraceAddEvent will reset stack0/mset0 in the new part for us.
+  TraceAddEvent(thr, thr->fast_state, EventTypeMop, 0);
+
   thr->fast_synch_epoch = epoch0;
   AcquireImpl(thr, 0, &sync);
-  thr->fast_state.SetHistorySize(flags()->history_size);
-  const uptr trace = (epoch0 / kTracePartSize) % TraceParts();
-  Trace *thr_trace = ThreadTrace(thr->tid);
-  TraceHeader *hdr = &thr_trace->headers[trace];
-  hdr->epoch0 = epoch0;
-  ObtainCurrentStack(thr, 0, &hdr->stack0);
-  hdr->mset0 = thr->mset;
   StatInc(thr, StatSyncAcquire);
   sync.Reset(&thr->clock_cache);
   DPrintf("#%d: ThreadStart epoch=%zu stk_addr=%zx stk_size=%zx "