tsan: increase max number of threads supported by test-only barrier

[dvyukov]

Summary

Currently the barrier supports only 256 threads,
this does not allow to write reliable tests that use more threads.
Bump max number of threads to 1024 to support writing
good stress tests.
Also replace sched_yield() with usleep(100) on the wait path.
If we write tests that create hundreds of threads (and dozens
of tests can run in parallel), yield would consume massive
amounts of CPU time for spinning.

Depends on D106952.

[melver]

compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp

2923

static constexpr ?

[dvyukov]

compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp

2923

3.5/3
A name having namespace scope (3.3.6) has internal linkage if it is the name of
— a variable that is explicitly declared const or constexpr and neither explicitly declared extern nor
previously declared to have external linkage;

[melver]

compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp

2923

Ah, very good.

compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp

 // 2. Then we switched to atomics+usleep(10). But usleep produced parasitic
 //    "as-if synchronized via sleep" messages in reports which failed some
 //    output tests.
 // 3. Then we switched to atomics+sched_yield. But this produced tons of tsan-
 //    visible events, which lead to "failed to restore stack trace" failures.
 // Note that no_sanitize_thread attribute does not turn off atomic interception
 // so attaching it to the function defined in user code does not help.
 // That's why we now have what we have.
+constexpr uptr kBarrierThreadBits = 10;
+constexpr uptr kBarrierThreads = 1 << kBarrierThreadBits;
+
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
 void __tsan_testonly_barrier_init(u64 *barrier, u32 count) {
-  if (count >= (1 << 8)) {
-      Printf("barrier_init: count is too large (%d)\n", count);
-      Die();
+  if (count >= kBarrierThreads) {
   }
-  // 8 lsb is thread count, the remaining are count of entered threads.
+  // kBarrierThreadBits lsb is thread count,
+  // the remaining are count of entered threads.
   *barrier = count;
 }
 
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
 void __tsan_testonly_barrier_wait(u64 *barrier) {
-  unsigned old = __atomic_fetch_add(barrier, 1 << 8, __ATOMIC_RELAXED);
-  unsigned old_epoch = (old >> 8) / (old & 0xff);
+  constexpr uptr kThreadMask = kBarrierThreads - 1;
+  unsigned old = __atomic_fetch_add(barrier, kBarrierThreads, __ATOMIC_RELAXED);
+  unsigned old_epoch = (old >> kBarrierThreadBits) / (old & kThreadMask);
   for (;;) {
     unsigned cur = __atomic_load_n(barrier, __ATOMIC_RELAXED);
-    unsigned cur_epoch = (cur >> 8) / (cur & 0xff);
+    unsigned cur_epoch = (cur >> kBarrierThreadBits) / (cur & kThreadMask);
     if (cur_epoch != old_epoch)
-      return;
-    internal_sched_yield();
+      break;
+    internal_usleep(100);
   }
 }