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Here we provide a high-level description of ThreadSanitizer (version 2) algorithm. The tool is work-in-progress, so some details may change in future. This is the second version of ThreadSanitizer, the first (slow and deprecated) version can be found here.

ThreadSanitizer consists of two parts: instrumentation module and a run-time library.


We instrument every memory access in the program unless it can be proven to be race-free or redundant. Memory accesses are simply prepended with a function call like __tsan_read4(addr).

Examples of race-free access:

  • Reads from constant globals (including vtables).
  • Accesses to memory that does not escape the current function (and therefore the current thread).

Examples of redundant accesses:

  • Read that happens before a write to the same location.

Atomic memory accesses are instrumented using specialized __tsan_atomic_ callbacks. Reads from vtable pointer are instrumented using __tsan_vptr_update to deal with benign vptr races. Function entry and exit are instrumented with __tsan_func_entry(caller_pc) and __tsan_func_exit. A call to __tsan_init is inserted before all initializers.

Run-time Library

Shadow State

Shadow State is N Shadow Words (described below); N is one of 2, 4, 8 (configurable). Every aligned 8-byte word of application memory is mapped into N Shadow Words using direct address mapping (no memory accesses required to compute the shadow address).

Shadow Word is a 64-bit object that contains the following fields:

TID (Thread Id) 16 bits (configurable)
Scalar Clock 42 bits (configurable)
IsWrite 1 bit
Access Size (1, 2, 4 or 8) 2 bits
Address Offset (0..7) 3 bits

One Shadow Word represents a single memory access to a subset of bytes within the 8-byte word of application memory. Therefore the Shadow State describes N different accesses to the corresponding application memory region.

State Machine

The core of the algorithm is the State Machine, a function that updates the Shadow State on every memory access.

First, the thread's clock is incremented and a new Shadow Word that corresponds to the current memory access is created. Then the State Machine iterates over all Shadow Words stored in the Shadow State. If one of the old Shadow Words constitutes a race with the new Shadow Word, a warning message is printed. The new Shadow Word is inserted in place of an empty Shadow Word or in place of a Shadow Word that happened-before the new one. If no place for insertion is found, a random Shadow Word is evicted.

All accesses to Shadow Words are 64-bit atomic loads/stores, but otherwise no locking is involved. Even if two threads are modifying the same Shadow State at the same time, the Shadow State will remain consistent. There is tiny probability to miss a data race though.

Approximate pseudo code follows (for an exact algorithm see the code in; we will update the pseudo code once the actual code is frozen).

def HandleMemoryAccess(addr, tid, is_write, size, pc):
  shadow_address = MapApplicationToShadow(addr)
  LogEvent(tid, pc);
  new_shadow_word = {tid, CurrentClock(tid), is_write, size, addr & 7}
  store_word = new_shadow_word
  for i in 1..N:
    UpdateOneShadowState(shadow_address, i, new_shadow_word, store_word)
  if store_word:
    # Evict a random Shadow Word
    shadow_address[Random(N)] = store_word  # Atomic
def UpdateOneShadowState(shadow_address, i, new_shadow_word, store_word):
  idx = (i + new_shadow_word.offset) % N
  old_shadow_word = shadow_address[idx]  # Atomic
  if old_shadow_word == 0: # The old state is empty
    if store_word:
      StoreIfNotYetStored(shadow_address[idx], store_word)
  if AccessedSameRegion(old_shadow_word, new_shadow_word):
    if SameThreads(old_shadow_word, new_shadow_word):
    else:  # Different threads
      if not HappensBefore(old_shadow_word, new_shadow_word):
        ReportRace(old_shadow_word, new_shadow_word)
  elif AccessedIntersectingRegions(old_shadow_word, new_shadow_word):
    if not SameThreads(old_shadow_word, new_shadow_word)
      if not HappensBefore(old_shadow_word, new_shadow_word)
        ReportRace(old_shadow_word, new_shadow_word)
  else: # regions did not intersect
    pass # do nothing

def StoreIfNotYetStored(shadow_address, store_word):
  *shadow_address = store_word  # Atomic
  store_word = 0

Event Trace

ThreadSanitizer needs to report stack traces and other information for previous accesses involved in a data race. LogEvent(tid, pc) (see above) stores the access event in a large thread-local circular buffer for later recovery. TODO: add more detail.

GNU Tools Cauldron 2012 Presentation

Finding races and memory errors with compiler instrumentation