systemclock: Use `futex_time64` syscall if available (32-bit systems) and use correct...

See also https://gitlab.gnome.org/GNOME/glib/-/issues/2634

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1648

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3561>

diff --git a/subprojects/gstreamer/gst/gstsystemclock.c b/subprojects/gstreamer/gst/gstsystemclock.c
index 6d0b6ec..7d8efb1 100644 (file)
--- a/subprojects/gstreamer/gst/gstsystemclock.c
+++ b/subprojects/gstreamer/gst/gstsystemclock.c
@@ -70,11 +70,15 @@
 #define GST_SYSTEM_CLOCK_WAIT(clock)            g_cond_wait(GST_SYSTEM_CLOCK_GET_COND(clock),GST_SYSTEM_CLOCK_GET_LOCK(clock))
 #define GST_SYSTEM_CLOCK_BROADCAST(clock)       g_cond_broadcast(GST_SYSTEM_CLOCK_GET_COND(clock))
 
-#if defined(HAVE_FUTEX)
+#if defined(HAVE_FUTEX) || defined(HAVE_FUTEX_TIME64)
 #include <unistd.h>
 #include <linux/futex.h>
 #include <sys/syscall.h>
 
+#if !defined(__NR_futex) && !defined(__NR_futex_time64)
+#error "Neither __NR_futex nor __NR_futex_time64 are defined but were found by meson"
+#endif
+
 #ifndef FUTEX_WAIT_BITSET_PRIVATE
 #define FUTEX_WAIT_BITSET_PRIVATE FUTEX_WAIT_BITSET
 #endif
@@ -123,14 +127,35 @@ gst_futex_cond_broadcast (guint * cond_val)
 {
   g_atomic_int_inc (cond_val);
 
+#if defined(__NR_futex_time64)
+  {
+    int res;
+    res = syscall (__NR_futex_time64, cond_val, (gsize) FUTEX_WAKE_PRIVATE,
+        (gsize) INT_MAX, NULL);
+
+    /* If the syscall does not exist (`ENOSYS`), we retry again below with the
+     * normal `futex` syscall. This can happen if newer kernel headers are
+     * used than the kernel that is actually running.
+     */
+#ifdef __NR_futex
+    if (res >= 0 || errno != ENOSYS) {
+#else
+    {
+#endif
+      return;
+    }
+  }
+#endif
+
+#if defined(__NR_futex)
   syscall (__NR_futex, cond_val, (gsize) FUTEX_WAKE_PRIVATE, (gsize) INT_MAX,
       NULL);
+#endif
 }
 
 static gboolean
 gst_futex_cond_wait_until (guint * cond_val, GMutex * mutex, gint64 end_time)
 {
-  struct timespec end;
   guint sampled;
   int res;
   gboolean success;
@@ -138,20 +163,92 @@ gst_futex_cond_wait_until (guint * cond_val, GMutex * mutex, gint64 end_time)
   if (end_time < 0)
     return FALSE;
 
-  end.tv_sec = end_time / 1000000000;
-  end.tv_nsec = end_time % 1000000000;
-
   sampled = *cond_val;
   g_mutex_unlock (mutex);
-  /* we use FUTEX_WAIT_BITSET_PRIVATE rather than FUTEX_WAIT_PRIVATE to be
-   * able to use absolute time */
-  res =
-      syscall (__NR_futex, cond_val, (gsize) FUTEX_WAIT_BITSET_PRIVATE,
-      (gsize) sampled, &end, NULL, FUTEX_BITSET_MATCH_ANY);
-  success = (res < 0 && errno == ETIMEDOUT) ? FALSE : TRUE;
-  g_mutex_lock (mutex);
-
-  return success;
+
+  /* `struct timespec` as defined by the libc headers does not necessarily
+   * have any relation to the one used by the kernel for the `futex` syscall.
+   *
+   * Specifically, the libc headers might use 64-bit `time_t` while the kernel
+   * headers use 32-bit `__kernel_old_time_t` on certain systems.
+   *
+   * To get around this problem we
+   *   a) check if `futex_time64` is available, which only exists on 32-bit
+   *      platforms and always uses 64-bit `time_t`.
+   *   b) otherwise (or if that returns `ENOSYS`), we call the normal `futex`
+   *      syscall with the `struct timespec_t` used by the kernel, which uses
+   *      `__kernel_long_t` for both its fields. We use that instead of
+   *      `__kernel_old_time_t` because it is equivalent and available in the
+   *      kernel headers for a longer time.
+   *
+   * Also some 32-bit systems do not define `__NR_futex` at all and only
+   * define `__NR_futex_time64`.
+   */
+
+#ifdef __NR_futex_time64
+  {
+    struct
+    {
+      gint64 tv_sec;
+      gint64 tv_nsec;
+    } end;
+
+    end.tv_sec = end_time / 1000000000;
+    end.tv_nsec = end_time % 1000000000;
+
+    /* we use FUTEX_WAIT_BITSET_PRIVATE rather than FUTEX_WAIT_PRIVATE to be
+     * able to use absolute time */
+    res =
+        syscall (__NR_futex_time64, cond_val, (gsize) FUTEX_WAIT_BITSET_PRIVATE,
+        (gsize) sampled, &end, NULL, FUTEX_BITSET_MATCH_ANY);
+
+    /* If the syscall does not exist (`ENOSYS`), we retry again below with the
+     * normal `futex` syscall. This can happen if newer kernel headers are
+     * used than the kernel that is actually running.
+     */
+#ifdef __NR_futex
+    if (res >= 0 || errno != ENOSYS) {
+#else
+    {
+#endif
+      success = (res < 0 && errno == ETIMEDOUT) ? FALSE : TRUE;
+      g_mutex_lock (mutex);
+
+      return success;
+    }
+  }
+#endif
+
+#ifdef __NR_futex
+  {
+    struct
+    {
+      __kernel_long_t tv_sec;
+      __kernel_long_t tv_nsec;
+    } end;
+
+    /* Make sure to only ever call this if the end time actually fits into the
+     * target type */
+    g_assert (sizeof (__kernel_long_t) >= 8
+        || end_time / 1000000000 <= G_MAXINT32);
+
+    end.tv_sec = end_time / 1000000000;
+    end.tv_nsec = end_time % 1000000000;
+
+    /* we use FUTEX_WAIT_BITSET_PRIVATE rather than FUTEX_WAIT_PRIVATE to be
+     * able to use absolute time */
+    res =
+        syscall (__NR_futex, cond_val, (gsize) FUTEX_WAIT_BITSET_PRIVATE,
+        (gsize) sampled, &end, NULL, FUTEX_BITSET_MATCH_ANY);
+    success = (res < 0 && errno == ETIMEDOUT) ? FALSE : TRUE;
+    g_mutex_lock (mutex);
+
+    return success;
+  }
+#endif
+
+  /* We can't end up here because of the checks above */
+  g_assert_not_reached ();
 }
 
 #elif defined (G_OS_UNIX)

diff --git a/subprojects/gstreamer/meson.build b/subprojects/gstreamer/meson.build
index 0641f76..7331c66 100644 (file)
--- a/subprojects/gstreamer/meson.build
+++ b/subprojects/gstreamer/meson.build
@@ -294,7 +294,7 @@ if cc.has_header_symbol('pthread.h', 'pthread_cond_timedwait_relative_np')
 endif
 
 # Check for futex(2)
-if cc.links('''#include <linux/futex.h>
+if cc.compiles('''#include <linux/futex.h>
                #include <sys/syscall.h>
                #include <unistd.h>
                int main (int argc, char ** argv) {
@@ -303,6 +303,15 @@ if cc.links('''#include <linux/futex.h>
                }''', name : 'futex(2) system call')
   cdata.set('HAVE_FUTEX', 1)
 endif
+if cc.compiles('''#include <linux/futex.h>
+               #include <sys/syscall.h>
+               #include <unistd.h>
+               int main (int argc, char ** argv) {
+                 syscall (__NR_futex_time64, NULL, FUTEX_WAKE, FUTEX_WAIT);
+                 return 0;
+               }''', name : 'futex(2) system call')
+  cdata.set('HAVE_FUTEX_TIME64', 1)
+endif
 
 # Check for posix timers and monotonic clock
 time_prefix = '#include <time.h>\n'