Race condition in steady_clock::now for _LIBCPP_WIN32API

[ivafanas]

Bugzilla Link	41323
Resolution	FIXED
Resolved on	Apr 02, 2019 07:00
Version	unspecified
OS	All
CC	@cpplearner,@ivafanas,@mclow

Extended Description

Method implementation is:

steady_clock::time_point
steady_clock::now() _NOEXCEPT
{
  static LARGE_INTEGER freq;
  static BOOL initialized = FALSE;
  if (!initialized)
    initialized = QueryPerformanceFrequency(&freq); // always succceeds

  LARGE_INTEGER counter;
  QueryPerformanceCounter(&counter);
  return time_point(duration(counter.QuadPart * nano::den / freq.QuadPart));
}

And seems like there is a race condition on freq and initialized variables if steady_clock::now has never been called before new threads creation.

Possible fixes are:

1. use thread local storage for freq and initialized, but there might be some problems on Windows XP:
   https://reverseengineering.stackexchange.com/questions/14171/thread-local-storage-access-on-windows-xp

2. use std::call_once guard, but it is slightly slower than thread local due to atomic access.

[mclow]

This should fix the problem, but I don't have direct access to a Windows box to check.
Can you let me know?
Also, how did you discover this?

diff --git a/libcxx/src/chrono.cpp b/libcxx/src/chrono.cpp
index ea0a638acc5..d342ee32d6c 100644
--- a/libcxx/src/chrono.cpp
+++ b/libcxx/src/chrono.cpp
@@ -177,16 +177,21 @@ steady_clock::now() _NOEXCEPT

#elif defined(_LIBCPP_WIN32API)

+static LARGE_INTEGER
+init_FREQ()
+{

• LARGE_INTEGER val;
• (void) QueryPerformanceFrequency(&val);
• return val;
  +}

steady_clock::time_point
steady_clock::now() _NOEXCEPT
{

• static LARGE_INTEGER freq;
• static BOOL initialized = FALSE;
• if (!initialized)
• initialized = QueryPerformanceFrequency(&freq); // always succceeds

• static const LARGE_INTEGER freq = init_FREQ();

  LARGE_INTEGER counter;

• QueryPerformanceCounter(&counter);

• (void) QueryPerformanceCounter(&counter);
  return time_point(duration(counter.QuadPart * nano::den / freq.QuadPart));
  }

[ivafanas]

Yes, this fix is ok.
And it is cleaner than my proposals.

Can you let me know? Also, how did you discover this?

Well, I do not have windows box too. I'm just reviewing libcpp code in order to understand how it is organized.

[mclow]

Fixed in r357413.

[cpplearner]

Fixed in r357413.

r357413 changes a invocation of "QueryPerformanceCounter" to instead call "QueryPerformanceFrequency" (note Counter vs. Frequency). Seems like a mistake?

[mclow]

Fixed in r357413.

r357413 changes a invocation of "QueryPerformanceCounter" to instead call
"QueryPerformanceFrequency" (note Counter vs. Frequency). Seems like a
mistake?

Oops. r357474