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glib/gthread-posix: Conditionally use futex and/or futex_time64 syscalls as necessary and use the correct struct timespec definition

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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.
This commit is contained in:
Sebastian Dröge 2022-12-12 19:58:21 +02:00
parent f67e8636da
commit a79c6af23e
4 changed files with 137 additions and 56 deletions
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15
glib/gbitlock.c
View File

@ -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

120
glib/gthread-posix.c
View File

@ -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

49
glib/gthreadprivate.h
View File

@ -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
{

9
meson.build
View File

@ -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>