[gcc r11-3153] libstdc++: Use FUTEX_CLOCK_REALTIME for futex wait

[Jonathan Wakely]

https://gcc.gnu.org/g:5bad23ceec0bfc9fea7c3da10b187366052de369

commit r11-3153-g5bad23ceec0bfc9fea7c3da10b187366052de369
Author: Mike Crowe <mac@mcrowe.com>
Date:   Fri Sep 11 14:24:59 2020 +0100

    libstdc++: Use FUTEX_CLOCK_REALTIME for futex wait
    
    The futex system call supports waiting for an absolute time if
    FUTEX_WAIT_BITSET is used rather than FUTEX_WAIT.  Doing so provides two
    benefits:
    
    1. The call to gettimeofday is not required in order to calculate a
       relative timeout.
    
    2. If someone changes the system clock during the wait then the futex
       timeout will correctly expire earlier or later.  Currently that only
       happens if the clock is changed prior to the call to gettimeofday.
    
    According to futex(2), support for FUTEX_CLOCK_REALTIME was added in the
    v2.6.28 Linux kernel and FUTEX_WAIT_BITSET was added in v2.6.25.  To
    ensure that the code still works correctly with earlier kernel versions,
    an ENOSYS error from futex[1] results in the
    futex_clock_realtime_unavailable flag being set.  This flag is used to
    avoid the unnecessary unsupported futex call in the future and to fall
    back to the previous gettimeofday and relative time implementation.
    
    glibc applied an equivalent switch in pthread_cond_timedwait to use
    FUTEX_CLOCK_REALTIME and FUTEX_WAIT_BITSET rather than FUTEX_WAIT for
    glibc-2.10 back in 2009.  See
    glibc:cbd8aeb836c8061c23a5e00419e0fb25a34abee7
    
    The futex_clock_realtime_unavailable flag is accessed using
    std::memory_order_relaxed to stop it becoming a bottleneck.  If the
    first two calls to _M_futex_wait_until happen to happen simultaneously
    then the only consequence is that both will try to use
    FUTEX_CLOCK_REALTIME, both risk discovering that it doesn't work and, if
    so, both set the flag.
    
    [1] This is how glibc's nptl-init.c determines whether these flags are
        supported.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++11/futex.cc: Add new constants for required futex
            flags.  Add futex_clock_realtime_unavailable flag to store
            result of trying to use FUTEX_CLOCK_REALTIME.
            (__atomic_futex_unsigned_base::_M_futex_wait_until): Try to
            use FUTEX_WAIT_BITSET with FUTEX_CLOCK_REALTIME and only
            fall back to using gettimeofday and FUTEX_WAIT if that's not
            supported.

Diff:
---
 libstdc++-v3/src/c++11/futex.cc | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/libstdc++-v3/src/c++11/futex.cc b/libstdc++-v3/src/c++11/futex.cc
index c9de11a7ec7..25b3e05d0d0 100644
--- a/libstdc++-v3/src/c++11/futex.cc
+++ b/libstdc++-v3/src/c++11/futex.cc
@@ -35,8 +35,16 @@
 
 // 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_realtime_flag = 256;
+const unsigned futex_bitset_match_any = ~0;
 const unsigned futex_wake_op = 1;
 
+namespace
+{
+  std::atomic<bool> futex_clock_realtime_unavailable;
+}
+
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
@@ -58,6 +66,35 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
     else
       {
+	if (!futex_clock_realtime_unavailable.load(std::memory_order_relaxed))
+	  {
+	    struct timespec rt;
+	    rt.tv_sec = __s.count();
+	    rt.tv_nsec = __ns.count();
+	    if (syscall (SYS_futex, __addr,
+			 futex_wait_bitset_op | futex_clock_realtime_flag,
+			 __val, &rt, nullptr, futex_bitset_match_any) == -1)
+	      {
+		__glibcxx_assert(errno == EINTR || errno == EAGAIN
+				|| errno == ETIMEDOUT || errno == ENOSYS);
+		if (errno == ETIMEDOUT)
+		  return false;
+		if (errno == ENOSYS)
+		  {
+		    futex_clock_realtime_unavailable.store(true,
+						    std::memory_order_relaxed);
+		    // Fall through to legacy implementation if the system
+		    // call is unavailable.
+		  }
+		else
+		  return true;
+	      }
+	    else
+	      return true;
+	  }
+
+	// We only get to here if futex_clock_realtime_unavailable was
+	// true or has just been set to true.
 	struct timeval tv;
 	gettimeofday (&tv, NULL);
 	// Convert the absolute timeout value to a relative timeout