[PATCH 0/5] Make std::future::wait_* use std::chrono::steady_clock when required

[Mike Crowe]

On Sunday 14 January 2018 at 16:08:09 +0000, Mike Crowe wrote:
> Hi Torvald,
> 
> Thanks for reviewing this change.
> 
> On Saturday 13 January 2018 at 16:29:57 +0100, Torvald Riegel wrote:
> > On Sun, 2018-01-07 at 20:55 +0000, Mike Crowe wrote:
> > > This is a first attempt to make std::future::wait_until and
> > > std::future::wait_for make correct use of
> > > std::chrono::steady_clock/CLOCK_MONOTONIC. It also makes
> > > std::future::wait_until react to changes to CLOCK_REALTIME during the
> > > wait, but only when passed a std::chrono::system_clock time point.
> > 
> > I have comments on the design.
> > 
> > First, I don't think we should not change
> > __atomic_futex_unsigned_base::_M_futex_wait_until, as there's a risk
> > that we'll change behavior of existing applications that work as
> > expected.
> 
> I assume you mean "I don't think we should change" or "I think we should
> not change"... :-)
> 
> The only way I can see that behaviour will change for existing programs is
> when the system clock changes (i.e. when someone calls settimeofday.) In
> the existing code, the maximum wait time is fixed once gettimeofday is
> called to calculate the relative timeout. When using FUTEX_CLOCK_REALTIME,
> the maximum wait can change based on changes to the system clock after that
> point. It appears that glibc made this transition successfully and
> currently uses FUTEX_CLOCK_REALTIME. I think that the new behaviour is
> better than the old behaviour.
> 
> Or perhaps I've missed another possibility. Did you have another risk in
> mind?
> 
> > Instead, ISTM we should additionally expose the two options we have at
> > the level of futexes:
> > * Relative timeout using CLOCK_MONOTONIC
> > * Absolute timeout using CLOCK_REALTIME (which will fall back to the
> > former on old kernels, which is fine I think).
> > 
> > Then we do the following translations from functions that programs would
> > call to the new futex functions:
> > 
> > 1) wait_for is a loop in which we load the current time from the steady
> > clock, then call the relative futex wait, and if that returns for a
> > spurious reason (ie, neither timeout nor is the expected value present),
> > we reduce the prior relative amount by the difference between the time
> > before the futex wait and the current time.
> 
> If we're going to loop on a relative timeout it sounds safer to convert it
> to an absolute (steady clock) timeout. That way we won't risk increasing
> the timeout if the scheduler decides not to run us at an inopportune moment
> between waits. _M_load_when_equal_for already does this.
> 
> _M_load_and_test_until already has a loop for spurious wakeup. I think that
> it makes sense to only loop at one level. That loop relies on the timeout
> being absolute, which is why my _M_load_and_test_until_steady also uses an
> absolute timeout.
> 
> > 2) wait_until using the steady clock is a loop similar to wait_for, just
> > that we additionally compute the initial relative timeout.
> 
> Clearly an absolute wait can be implemented in terms of a relative one and
> vice-versa, but at least in my attempts to write them I find the code
> easier to understand (and therefore get right) if the fundamental wait is
> the absolute one and the relative one is implemented on top of it.

I had a quick go at implementing at least the first part of your design, as
I understood it. (I've kept the loops inside atomic_futex_unsigned - and I
think that you wanted to move them out to the client code.) I've not tested
it much.

I think that this implementation of _M_load_and_test_for is rather more
error-prone than my previous _M_load_and_test_until_steady. That's probably
partly because the type-safe duration has already been separated into seconds
and nanoseconds. It would be nice to push this separation as deeply as
possible in the code, but I'm afraid that would break ABI compatibility.

Thanks.

Mike.

--8<--

diff --git a/libstdc++-v3/include/bits/atomic_futex.h b/libstdc++-v3/include/bits/atomic_futex.h
index ad9437da4e2..fa4a4382c79 100644
--- a/libstdc++-v3/include/bits/atomic_futex.h
+++ b/libstdc++-v3/include/bits/atomic_futex.h
@@ -57,6 +57,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _M_futex_wait_until(unsigned *__addr, unsigned __val, bool __has_timeout,
 	chrono::seconds __s, chrono::nanoseconds __ns);

+    // Returns false iff a timeout occurred.
+    bool
+    _M_futex_wait_for(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);
   };
@@ -110,6 +115,40 @@ _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.
+    unsigned
+    _M_load_and_test_for(unsigned __assumed, unsigned __operand,
+	bool __equal, memory_order __mo, bool __has_timeout,
+	chrono::seconds __s, chrono::nanoseconds __ns)
+    {
+      auto __end_time = chrono::steady_clock::now() + 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_for((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;
+
+	  const auto __remaining = __end_time - chrono::steady_clock::now();
+	  __s = chrono::duration_cast<chrono::seconds>(__remaining);
+	  __ns = chrono::duration_cast<chrono::nanoseconds>(__remaining - __s);
+	}
+      return __assumed;
+    }
+
     // 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
@@ -168,8 +207,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _M_load_when_equal_for(unsigned __val, memory_order __mo,
 	  const chrono::duration<_Rep, _Period>& __rtime)
       {
-	return _M_load_when_equal_until(__val, __mo,
-					__clock_t::now() + __rtime);
+	unsigned __i = _M_load(__mo);
+	if ((__i & ~_Waiter_bit) == __val)
+	  return true;
+	auto __s = chrono::duration_cast<chrono::seconds>(__rtime);
+	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__rtime - __s);
+	__i = _M_load_and_test_for(__i, __val, true, __mo, true, __s, __ns);
+	return (__i & ~_Waiter_bit) == __val;
       }

     // Returns false iff a timeout occurred.
diff --git a/libstdc++-v3/src/c++11/futex.cc b/libstdc++-v3/src/c++11/futex.cc
index f5000aa77b3..1b5fe480209 100644
--- a/libstdc++-v3/src/c++11/futex.cc
+++ b/libstdc++-v3/src/c++11/futex.cc
@@ -84,6 +84,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
   }

+  bool
+  __atomic_futex_unsigned_base::_M_futex_wait_for(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_op, __val, &rt) == -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