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[RFC PATCH 2/4] libstdc++ futex: Support waiting on std::chrono::steady_clock directly


The user-visible effect of this change is for std::future::wait_until
to use CLOCK_MONOTONIC when passed a timeout of
std::chrono::steady_clock::time_point type. This makes it immune to
any changes made to the system clock CLOCK_REALTIME.

Add an overload of __atomic_futex_unsigned::_M_load_and_text_until_impl
that accepts a std::chrono::steady_clock, and correctly passes this through
to __atomic_futex_unsigned_base::_M_futex_wait_until_steady which uses
CLOCK_MONOTONIC for the timeout within the futex system call. These
functions are mostly just copies of the std::chrono::system_clock versions
with small tweaks.

Prior to this commit, a std::chrono::steady timeout would be converted via
std::chrono::system_clock which risks reducing or increasing the timeout if
someone changes CLOCK_REALTIME whilst the wait is happening. (The commit
immediately prior to this one increases the window of opportunity for that
from a short period during the calculation of a relative timeout, to the
entire duration of the wait.)

I believe that I've added this functionality in a way that it doesn't break
ABI compatibility, but that has made it more verbose and less type safe. I
believe that it would be better to maintain the timeout as an instance of
the correct clock type all the way down to a single _M_futex_wait_until
function with an overload for each clock. The current scheme of separating
out the seconds and nanoseconds early risks accidentally calling the wait
function for the wrong clock. Unfortunately, doing this would break code
that compiled against the old header.
---
 libstdc++-v3/include/bits/atomic_futex.h | 67 +++++++++++++++++++++++++++++++-
 libstdc++-v3/src/c++11/futex.cc          | 33 ++++++++++++++++
 2 files changed, 99 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/atomic_futex.h b/libstdc++-v3/include/bits/atomic_futex.h
index afcfeb7720d..c2b3df03592 100644
--- a/libstdc++-v3/include/bits/atomic_futex.h
+++ b/libstdc++-v3/include/bits/atomic_futex.h
@@ -52,11 +52,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1
   struct __atomic_futex_unsigned_base
   {
-    // Returns false iff a timeout occurred.
+    // __s and __ns are measured against CLOCK_REALTIME. Returns false
+    // iff a timeout occurred.
     bool
     _M_futex_wait_until(unsigned *__addr, unsigned __val, bool __has_timeout,
 	chrono::seconds __s, chrono::nanoseconds __ns);
 
+    // __s and __ns are measured against CLOCK_MONOTONIC. Returns
+    // false iff a timeout occurred.
+    bool
+    _M_futex_wait_until_steady(unsigned *__addr, unsigned __val, bool __has_timeout,
+	chrono::seconds __s, chrono::nanoseconds __ns);
+
     // This can be executed after the object has been destroyed.
     static void _M_futex_notify_all(unsigned* __addr);
   };
@@ -86,6 +93,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     // value if equal is false.
     // The assumed value is the caller's assumption about the current value
     // when making the call.
+    // __s and __ns are measured against CLOCK_REALTIME.
     unsigned
     _M_load_and_test_until(unsigned __assumed, unsigned __operand,
 	bool __equal, memory_order __mo, bool __has_timeout,
@@ -110,6 +118,36 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	}
     }
 
+    // If a timeout occurs, returns a current value after the timeout;
+    // otherwise, returns the operand's value if equal is true or a different
+    // value if equal is false.
+    // The assumed value is the caller's assumption about the current value
+    // when making the call.
+    // __s and __ns are measured against CLOCK_MONOTONIC.
+    unsigned
+    _M_load_and_test_until_steady(unsigned __assumed, unsigned __operand,
+	bool __equal, memory_order __mo, bool __has_timeout,
+	chrono::seconds __s, chrono::nanoseconds __ns)
+    {
+      for (;;)
+	{
+	  // Don't bother checking the value again because we expect the caller
+	  // to have done it recently.
+	  // memory_order_relaxed is sufficient because we can rely on just the
+	  // modification order (store_notify uses an atomic RMW operation too),
+	  // and the futex syscalls synchronize between themselves.
+	  _M_data.fetch_or(_Waiter_bit, memory_order_relaxed);
+	  bool __ret = _M_futex_wait_until_steady((unsigned*)(void*)&_M_data,
+					   __assumed | _Waiter_bit,
+					   __has_timeout, __s, __ns);
+	  // Fetch the current value after waiting (clears _Waiter_bit).
+	  __assumed = _M_load(__mo);
+	  if (!__ret || ((__operand == __assumed) == __equal))
+	    return __assumed;
+	  // TODO adapt wait time
+	}
+    }
+
     // Returns the operand's value if equal is true or a different value if
     // equal is false.
     // The assumed value is the caller's assumption about the current value
@@ -140,6 +178,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  true, __s.time_since_epoch(), __ns);
     }
 
+    template<typename _Dur>
+    unsigned
+    _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
+	bool __equal, memory_order __mo,
+	const chrono::time_point<std::chrono::steady_clock, _Dur>& __atime)
+    {
+      auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
+      auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
+      // XXX correct?
+      return _M_load_and_test_until_steady(__assumed, __operand, __equal, __mo,
+	  true, __s.time_since_epoch(), __ns);
+    }
+
   public:
 
     _GLIBCXX_ALWAYS_INLINE unsigned
@@ -200,6 +251,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return (__i & ~_Waiter_bit) == __val;
     }
 
+    // Returns false iff a timeout occurred.
+    template<typename _Duration>
+    _GLIBCXX_ALWAYS_INLINE bool
+    _M_load_when_equal_until(unsigned __val, memory_order __mo,
+	const chrono::time_point<std::chrono::steady_clock, _Duration>& __atime)
+    {
+      unsigned __i = _M_load(__mo);
+      if ((__i & ~_Waiter_bit) == __val)
+	return true;
+      // TODO Spin-wait first.  Ignore effect on timeout.
+      __i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
+      return (__i & ~_Waiter_bit) == __val;
+    }
+
     _GLIBCXX_ALWAYS_INLINE void
     _M_store_notify_all(unsigned __val, memory_order __mo)
     {
diff --git a/libstdc++-v3/src/c++11/futex.cc b/libstdc++-v3/src/c++11/futex.cc
index 217aeefe005..fc2dcd7ff99 100644
--- a/libstdc++-v3/src/c++11/futex.cc
+++ b/libstdc++-v3/src/c++11/futex.cc
@@ -36,6 +36,7 @@
 // Constants for the wait/wake futex syscall operations
 const unsigned futex_wait_op = 0;
 const unsigned futex_wait_bitset_op = 9;
+const unsigned futex_clock_monotonic_flag = 0;
 const unsigned futex_clock_realtime_flag = 256;
 const unsigned futex_bitset_match_any = ~0;
 const unsigned futex_wake_op = 1;
@@ -75,6 +76,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
   }
 
+  bool
+  __atomic_futex_unsigned_base::_M_futex_wait_until_steady(unsigned *__addr,
+      unsigned __val,
+      bool __has_timeout, chrono::seconds __s, chrono::nanoseconds __ns)
+  {
+    if (!__has_timeout)
+      {
+	// Ignore whether we actually succeeded to block because at worst,
+	// we will fall back to spin-waiting.  The only thing we could do
+	// here on errors is abort.
+	int ret __attribute__((unused));
+	ret = syscall (SYS_futex, __addr, futex_wait_op, __val, nullptr);
+	_GLIBCXX_DEBUG_ASSERT(ret == 0 || errno == EINTR || errno == EAGAIN);
+	return true;
+      }
+    else
+      {
+	struct timespec rt;
+	rt.tv_sec = __s.count();
+	rt.tv_nsec = __ns.count();
+
+	if (syscall (SYS_futex, __addr, futex_wait_bitset_op | futex_clock_monotonic_flag, __val, &rt, nullptr, futex_bitset_match_any) == -1)
+	  {
+	    _GLIBCXX_DEBUG_ASSERT(errno == EINTR || errno == EAGAIN
+				  || errno == ETIMEDOUT);
+	    if (errno == ETIMEDOUT)
+	      return false;
+	  }
+	return true;
+      }
+  }
+
   void
   __atomic_futex_unsigned_base::_M_futex_notify_all(unsigned* __addr)
   {
-- 
2.11.0


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