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From 9ca6b1196e31a4a483fdddad191221616e2c9c3d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Dr=C3=B6ge?= <sebastian@centricular.com>
Date: Mon, 12 Dec 2022 11:34:51 +0200
Subject: [PATCH] systemclock: Use `futex_time64` syscall if available (32-bit
systems) and use correct `struct timespec` definition
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>
---
gst/gstsystemclock.c | 125 ++++++++++++++++++---
meson.build | 11 +-
2 files changed, 121 insertions(+), 15 deletions(-)
diff --git a/gst/gstsystemclock.c b/gst/gstsystemclock.c
index 6d0b6ec47b..7d8efb171e 100644
--- a/gst/gstsystemclock.c
+++ b/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/meson.build b/meson.build
index 0641f7678b..7331c66150 100644
--- a/meson.build
+++ b/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'
--
2.25.1
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