From f67e8636dad3038b584887bd1edf4515fbd7ac4a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Dr=C3=B6ge?= <sebastian@centricular.com>
Date: Tue, 13 Dec 2022 15:05:06 +0200
Subject: [PATCH 1/2] glib/gthread-posix: Use `cc.compiles()` instead of
`cc.links()` for checking for `__NR_futex`
`cc.compiles()` is minimally faster.
We only want to check here whether `__NR_futex` is defined and don't
want to check anything at link-time.
---
meson.build | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/meson.build b/meson.build
index 2305b3a08e..c763857ea9 100644
--- a/meson.build
+++ b/meson.build
@@ -887,7 +887,7 @@ if host_system == 'qnx'
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) {
--
GitLab
From a79c6af23eff5ee978db62e048828c9a992a1261 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Dr=C3=B6ge?= <sebastian@centricular.com>
Date: Mon, 12 Dec 2022 19:58:21 +0200
Subject: [PATCH 2/2] glib/gthread-posix: Conditionally use `futex` and/or
`futex_time64` syscalls as necessary and use the correct `struct timespec`
definition
On some systems only `futex_time64` exists (e.g. riscv32) while on
others only `futex` exists (old Linux, 64 bit platforms), so it is
necessary to check for both and try calling both at runtime.
Additionally use the correct `struct timespec` definition. There is not
necessarily any relation between the libc's definition and the kernel's.
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` 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.
---
glib/gbitlock.c | 15 ++----
glib/gthread-posix.c | 120 ++++++++++++++++++++++++++----------------
glib/gthreadprivate.h | 49 +++++++++++++++++
meson.build | 9 ++++
4 files changed, 137 insertions(+), 56 deletions(-)
diff --git a/glib/gbitlock.c b/glib/gbitlock.c
index 81bfb339f3..9c34de80c8 100644
--- a/glib/gbitlock.c
+++ b/glib/gbitlock.c
@@ -35,6 +35,7 @@
#ifdef G_BIT_LOCK_FORCE_FUTEX_EMULATION
#undef HAVE_FUTEX
+#undef HAVE_FUTEX_TIME64
#endif
#ifndef HAVE_FUTEX
@@ -42,7 +43,7 @@ static GMutex g_futex_mutex;
static GSList *g_futex_address_list = NULL;
#endif
-#ifdef HAVE_FUTEX
+#if defined(HAVE_FUTEX) || defined(HAVE_FUTEX_TIME64)
/*
* We have headers for futex(2) on the build machine. This does not
* imply that every system that ever runs the resulting glib will have
@@ -51,14 +52,6 @@ static GSList *g_futex_address_list = NULL;
*
* If anyone actually gets bit by this, please file a bug. :)
*/
-#include <linux/futex.h>
-#include <sys/syscall.h>
-#include <unistd.h>
-
-#ifndef FUTEX_WAIT_PRIVATE
-#define FUTEX_WAIT_PRIVATE FUTEX_WAIT
-#define FUTEX_WAKE_PRIVATE FUTEX_WAKE
-#endif
/* < private >
* g_futex_wait:
@@ -81,7 +74,7 @@ static void
g_futex_wait (const gint *address,
gint value)
{
- syscall (__NR_futex, address, (gsize) FUTEX_WAIT_PRIVATE, (gsize) value, NULL);
+ g_futex_simple (address, (gsize) FUTEX_WAIT_PRIVATE, (gsize) value, NULL);
}
/* < private >
@@ -98,7 +91,7 @@ g_futex_wait (const gint *address,
static void
g_futex_wake (const gint *address)
{
- syscall (__NR_futex, address, (gsize) FUTEX_WAKE_PRIVATE, (gsize) 1, NULL);
+ g_futex_simple (address, (gsize) FUTEX_WAKE_PRIVATE, (gsize) 1, NULL);
}
#else
diff --git a/glib/gthread-posix.c b/glib/gthread-posix.c
index d96fca5ca2..84f62881d8 100644
--- a/glib/gthread-posix.c
+++ b/glib/gthread-posix.c
@@ -74,7 +74,7 @@
#include <sys/syscall.h>
#endif
-#if defined(HAVE_FUTEX) && \
+#if (defined(HAVE_FUTEX) || defined(HAVE_FUTEX_TIME64)) && \
(defined(HAVE_STDATOMIC_H) || defined(__ATOMIC_SEQ_CST))
#define USE_NATIVE_MUTEX
#endif
@@ -1397,15 +1397,6 @@ g_system_thread_set_name (const gchar *name)
/* {{{1 GMutex and GCond futex implementation */
#if defined(USE_NATIVE_MUTEX)
-
-#include <linux/futex.h>
-#include <sys/syscall.h>
-
-#ifndef FUTEX_WAIT_PRIVATE
-#define FUTEX_WAIT_PRIVATE FUTEX_WAIT
-#define FUTEX_WAKE_PRIVATE FUTEX_WAKE
-#endif
-
/* We should expand the set of operations available in gatomic once we
* have better C11 support in GCC in common distributions (ie: 4.9).
*
@@ -1500,8 +1491,8 @@ g_mutex_lock_slowpath (GMutex *mutex)
*/
while (exchange_acquire (&mutex->i[0], G_MUTEX_STATE_CONTENDED) != G_MUTEX_STATE_EMPTY)
{
- syscall (__NR_futex, &mutex->i[0], (gsize) FUTEX_WAIT_PRIVATE,
- G_MUTEX_STATE_CONTENDED, NULL);
+ g_futex_simple (&mutex->i[0], (gsize) FUTEX_WAIT_PRIVATE,
+ G_MUTEX_STATE_CONTENDED, NULL);
}
}
@@ -1519,7 +1510,7 @@ g_mutex_unlock_slowpath (GMutex *mutex,
g_abort ();
}
- syscall (__NR_futex, &mutex->i[0], (gsize) FUTEX_WAKE_PRIVATE, (gsize) 1, NULL);
+ g_futex_simple (&mutex->i[0], (gsize) FUTEX_WAKE_PRIVATE, (gsize) 1, NULL);
}
void
@@ -1587,7 +1578,7 @@ g_cond_wait (GCond *cond,
guint sampled = (guint) g_atomic_int_get (&cond->i[0]);
g_mutex_unlock (mutex);
- syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAIT_PRIVATE, (gsize) sampled, NULL);
+ g_futex_simple (&cond->i[0], (gsize) FUTEX_WAIT_PRIVATE, (gsize) sampled, NULL);
g_mutex_lock (mutex);
}
@@ -1596,7 +1587,7 @@ g_cond_signal (GCond *cond)
{
g_atomic_int_inc (&cond->i[0]);
- syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAKE_PRIVATE, (gsize) 1, NULL);
+ g_futex_simple (&cond->i[0], (gsize) FUTEX_WAKE_PRIVATE, (gsize) 1, NULL);
}
void
@@ -1604,7 +1595,7 @@ g_cond_broadcast (GCond *cond)
{
g_atomic_int_inc (&cond->i[0]);
- syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAKE_PRIVATE, (gsize) INT_MAX, NULL);
+ g_futex_simple (&cond->i[0], (gsize) FUTEX_WAKE_PRIVATE, (gsize) INT_MAX, NULL);
}
gboolean
@@ -1614,12 +1605,6 @@ g_cond_wait_until (GCond *cond,
{
struct timespec now;
struct timespec span;
-#ifdef __NR_futex_time64
- long span_arg[2];
- G_STATIC_ASSERT (sizeof (span_arg[0]) == 4);
-#else
- struct timespec span_arg;
-#endif
guint sampled;
int res;
@@ -1640,37 +1625,82 @@ g_cond_wait_until (GCond *cond,
if (span.tv_sec < 0)
return FALSE;
- /* On x32 (ILP32 ABI on x86_64) and potentially sparc64, the raw futex()
- * syscall takes a 32-bit timespan argument *regardless* of whether userspace
- * is using 32-bit or 64-bit `struct timespec`. This means that we can’t
- * unconditionally pass a `struct timespec` pointer into the syscall.
+ /* `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.
*
- * Assume that any such platform is new enough to define the
- * `__NR_futex_time64` workaround syscall (which accepts 64-bit timespecs,
- * introduced in kernel 5.1), and use that as a proxy for whether to pass in
- * `long[2]` or `struct timespec`.
+ * Specifically, the libc headers might use 64-bit `time_t` while the kernel
+ * headers use 32-bit `__kernel_old_time_t` on certain systems.
*
- * As per https://lwn.net/Articles/776427/, the `time64` syscalls only exist
- * on 32-bit platforms, so in this case `sizeof(long)` should always be
- * 32 bits.
+ * 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` 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.
*
- * Don’t bother actually calling `__NR_futex_time64` as the `span` is relative
- * and hence very unlikely to overflow, even if using 32-bit longs.
+ * Also some 32-bit systems do not define `__NR_futex` at all and only
+ * define `__NR_futex_time64`.
*/
-#ifdef __NR_futex_time64
- span_arg[0] = span.tv_sec;
- span_arg[1] = span.tv_nsec;
-#else
- span_arg = span;
-#endif
sampled = cond->i[0];
g_mutex_unlock (mutex);
- res = syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAIT_PRIVATE, (gsize) sampled, &span_arg);
- success = (res < 0 && errno == ETIMEDOUT) ? FALSE : TRUE;
- g_mutex_lock (mutex);
- return success;
+#ifdef __NR_futex_time64
+ {
+ struct
+ {
+ gint64 tv_sec;
+ gint64 tv_nsec;
+ } span_arg;
+
+ span_arg.tv_sec = span.tv_sec;
+ span_arg.tv_nsec = span.tv_nsec;
+
+ res = syscall (__NR_futex_time64, &cond->i[0], (gsize) FUTEX_WAIT_PRIVATE, (gsize) sampled, &span_arg);
+
+ /* 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)
+# endif /* defined(__NR_futex) */
+ {
+ 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;
+ } span_arg;
+
+ /* Make sure to only ever call this if the end time actually fits into the target type */
+ if (G_UNLIKELY (sizeof (__kernel_long_t) < 8 && span.tv_sec > G_MAXINT32))
+ g_error ("%s: Can’t wait for more than %us", G_STRFUNC, G_MAXINT32);
+
+ span_arg.tv_sec = span.tv_sec;
+ span_arg.tv_nsec = span.tv_nsec;
+
+ res = syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAIT_PRIVATE, (gsize) sampled, &span_arg);
+ success = (res < 0 && errno == ETIMEDOUT) ? FALSE : TRUE;
+ g_mutex_lock (mutex);
+
+ return success;
+ }
+#endif /* defined(__NR_futex) */
+
+ /* We can't end up here because of the checks above */
+ g_assert_not_reached ();
}
#endif
diff --git a/glib/gthreadprivate.h b/glib/gthreadprivate.h
index 2ae705d4f5..6eaf422753 100644
--- a/glib/gthreadprivate.h
+++ b/glib/gthreadprivate.h
@@ -40,6 +40,55 @@ struct _GRealThread
/* system thread implementation (gthread-posix.c, gthread-win32.c) */
+#if defined(HAVE_FUTEX) || defined(HAVE_FUTEX_TIME64)
+#include <linux/futex.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#ifndef FUTEX_WAIT_PRIVATE
+#define FUTEX_WAIT_PRIVATE FUTEX_WAIT
+#define FUTEX_WAKE_PRIVATE FUTEX_WAKE
+#endif
+
+/* Wrapper macro to call `futex_time64` and/or `futex` with simple
+ * parameters and without returning the return value.
+ *
+ * If the `futex_time64` syscall does not exist (`ENOSYS`), we retry again
+ * with the normal `futex` syscall. This can happen if newer kernel headers
+ * are used than the kernel that is actually running.
+ *
+ * This must not be called with a timeout parameter as that differs
+ * in size between the two syscall variants!
+ */
+#if defined(__NR_futex) && defined(__NR_futex_time64)
+#define g_futex_simple(uaddr, futex_op, ...) \
+ G_STMT_START \
+ { \
+ int res = syscall (__NR_futex_time64, uaddr, (gsize) futex_op, __VA_ARGS__); \
+ if (res < 0 && errno == ENOSYS) \
+ syscall (__NR_futex, uaddr, (gsize) futex_op, __VA_ARGS__); \
+ } \
+ G_STMT_END
+#elif defined(__NR_futex_time64)
+#define g_futex_simple(uaddr, futex_op, ...) \
+ G_STMT_START \
+ { \
+ syscall (__NR_futex_time64, uaddr, (gsize) futex_op, __VA_ARGS__); \
+ } \
+ G_STMT_END
+#elif defined(__NR_futex)
+#define g_futex_simple(uaddr, futex_op, ...) \
+ G_STMT_START \
+ { \
+ syscall (__NR_futex, uaddr, (gsize) futex_op, __VA_ARGS__); \
+ } \
+ G_STMT_END
+#else /* !defined(__NR_futex) && !defined(__NR_futex_time64) */
+#error "Neither __NR_futex nor __NR_futex_time64 are defined but were found by meson"
+#endif /* defined(__NR_futex) && defined(__NR_futex_time64) */
+
+#endif
+
/* Platform-specific scheduler settings for a thread */
typedef struct
{
diff --git a/meson.build b/meson.build
index c763857ea9..75a3db504e 100644
--- a/meson.build
+++ b/meson.build
@@ -896,6 +896,15 @@ if cc.compiles('''#include <linux/futex.h>
}''', name : 'futex(2) system call')
glib_conf.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')
+ glib_conf.set('HAVE_FUTEX_TIME64', 1)
+endif
# Check for eventfd(2)
if cc.links('''#include <sys/eventfd.h>
--
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