futex: Implement FUTEX_WAIT_MULTIPLE

This is a new futex operation to allow a thread to wait on several
futexes at the same time, and wake up on any of them.  In a sense, it
implements one of the features that was supported by pooling on the old
FUTEX_FD interface.

My use case for this feature lies in Wine, where we want to implement a
similar function available in Windows, mainly for event handling.  The
wine folks have an implementation of the userspace side using eventfd,
but it suffers from bad performance, as shown in the measurements below.

Technically, the old FUTEX_WAIT implementation can be easily
reimplemented using do_futex_wait_multiple, with a count one, and I have
a patch demonstrating how it works.  I'm not proposing it, since futex
is such a tricky code, that I'd be more confortable to have
FUTEX_WAIT_MULTIPLE running upstream for a couple development cycles,
before considering modifying FUTEX_WAIT.

This was tested using three mechanisms:

1) By reimplementing FUTEX_WAIT in terms of FUTEX_WAIT_MULTIPLE and
running tools/testing/selftests/futex and a full linux distro on top of
this kernel.

2) By an example code that exercises the FUTEX_WAIT_MULTIPLE path on a
multi thread, event handling setup.

3) By running the Wine fsync implementation and executing multi-threaded
applications, in particular modern games on top of the implementation.
Signed-off-by: default avatarZebediah Figura <z.figura12@gmail.com>
Signed-off-by: default avatarSteven Noonan <steven@valvesoftware.com>
Signed-off-by: Pierre-Loup Griffais's avatarPierre-Loup A. Griffais <pgriffais@valvesoftware.com>
Signed-off-by: default avatarGabriel Krisman Bertazi <krisman@collabora.com>
parent a26ac2ef
......@@ -21,6 +21,7 @@
#define FUTEX_WAKE_BITSET 10
#define FUTEX_WAIT_REQUEUE_PI 11
#define FUTEX_CMP_REQUEUE_PI 12
#define FUTEX_WAIT_MULTIPLE 13
#define FUTEX_PRIVATE_FLAG 128
#define FUTEX_CLOCK_REALTIME 256
......@@ -150,4 +151,10 @@ struct robust_list_head {
(((op & 0xf) << 28) | ((cmp & 0xf) << 24) \
| ((oparg & 0xfff) << 12) | (cmparg & 0xfff))
struct futex_wait_block {
__u32 __user *uaddr;
__u32 val;
__u32 bitset;
};
#endif /* _UAPI_LINUX_FUTEX_H */
......@@ -183,6 +183,7 @@ static int __read_mostly futex_cmpxchg_enabled;
#endif
#define FLAGS_CLOCKRT 0x02
#define FLAGS_HAS_TIMEOUT 0x04
#define FLAGS_WAKE_MULTIPLE 0x08
/*
* Priority Inheritance state:
......@@ -2720,6 +2721,148 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
return ret;
}
static int do_futex_wait_multiple(struct futex_wait_block *wb,
u32 count, unsigned int flags,
ktime_t *abs_time)
{
struct hrtimer_sleeper timeout, *to;
struct futex_hash_bucket *hb;
struct futex_q *qs = NULL;
int ret;
int i;
qs = kcalloc(count, sizeof(struct futex_q), GFP_KERNEL);
if (!qs)
return -ENOMEM;
to = futex_setup_timer(abs_time, &timeout, flags,
current->timer_slack_ns);
retry:
for (i = 0; i < count; i++) {
qs[i].key = FUTEX_KEY_INIT;
qs[i].bitset = wb[i].bitset;
ret = get_futex_key(wb[i].uaddr, flags & FLAGS_SHARED,
&qs[i].key, FUTEX_READ);
if (unlikely(ret != 0)) {
for (--i; i >= 0; i--)
put_futex_key(&qs[i].key);
goto out;
}
}
set_current_state(TASK_INTERRUPTIBLE);
for (i = 0; i < count; i++) {
ret = __futex_wait_setup(wb[i].uaddr, wb[i].val,
flags, &qs[i], &hb);
if (ret) {
/* Drop the failed key directly. keys 0..(i-1)
* will be put by unqueue_me. */
put_futex_key(&qs[i].key);
/* Undo the partial work we did. */
for (--i; i >= 0; i--)
unqueue_me(&qs[i]);
__set_current_state(TASK_RUNNING);
if (ret > 0)
goto retry;
goto out;
}
/* We can't hold to the bucket lock when dealing with
* the next futex. Queue ourselves now so we can unlock
* it before moving on. */
queue_me(&qs[i], hb);
}
if (to)
hrtimer_start_expires(&to->timer, HRTIMER_MODE_ABS);
/* There is no easy to way to check if we are wake already on
* multiple futexes without waking through each one of them. So
* just sleep and let the scheduler handle it.
*/
if (!to || to->task)
freezable_schedule();
__set_current_state(TASK_RUNNING);
ret = -ETIMEDOUT;
/* If we were woken (and unqueued), we succeeded. */
for (i = 0; i < count; i++)
if (!unqueue_me(&qs[i]))
ret = i;
/* Succeed wakeup */
if (ret >= 0)
goto out;
/* Woken by triggered timeout */
if (to && !to->task)
goto out;
/*
* We expect signal_pending(current), but we might be the
* victim of a spurious wakeup as well.
*/
if (!signal_pending(current))
goto retry;
ret = -ERESTARTSYS;
if (!abs_time)
goto out;
ret = -ERESTART_RESTARTBLOCK;
out:
if (to) {
hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
}
kfree(qs);
return ret;
}
static int futex_wait_multiple(u32 __user *uaddr, unsigned int flags,
u32 count, ktime_t *abs_time)
{
struct futex_wait_block *wb;
struct restart_block *restart;
int ret;
if (!count)
return -EINVAL;
wb = kcalloc(count, sizeof(struct futex_wait_block), GFP_KERNEL);
if (!wb)
return -ENOMEM;
if (copy_from_user(wb, uaddr,
count * sizeof(struct futex_wait_block))) {
ret = -EFAULT;
goto out;
}
ret = do_futex_wait_multiple(wb, count, flags, abs_time);
if (ret == -ERESTART_RESTARTBLOCK) {
restart = &current->restart_block;
restart->fn = futex_wait_restart;
restart->futex.uaddr = uaddr;
restart->futex.val = count;
restart->futex.time = *abs_time;
restart->futex.flags = (flags | FLAGS_HAS_TIMEOUT |
FLAGS_WAKE_MULTIPLE);
}
out:
kfree(wb);
return ret;
}
static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
ktime_t *abs_time, u32 bitset)
{
......@@ -2797,6 +2940,10 @@ static long futex_wait_restart(struct restart_block *restart)
}
restart->fn = do_no_restart_syscall;
if (restart->futex.flags & FLAGS_WAKE_MULTIPLE)
return (long)futex_wait_multiple(uaddr, restart->futex.flags,
restart->futex.val, tp);
return (long)futex_wait(uaddr, restart->futex.flags,
restart->futex.val, tp, restart->futex.bitset);
}
......@@ -3680,6 +3827,8 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
uaddr2);
case FUTEX_CMP_REQUEUE_PI:
return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 1);
case FUTEX_WAIT_MULTIPLE:
return futex_wait_multiple(uaddr, flags, val, timeout);
}
return -ENOSYS;
}
......@@ -3696,7 +3845,8 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
cmd == FUTEX_WAIT_BITSET ||
cmd == FUTEX_WAIT_REQUEUE_PI)) {
cmd == FUTEX_WAIT_REQUEUE_PI ||
cmd == FUTEX_WAIT_MULTIPLE)) {
if (unlikely(should_fail_futex(!(op & FUTEX_PRIVATE_FLAG))))
return -EFAULT;
if (get_timespec64(&ts, utime))
......@@ -3705,7 +3855,7 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
return -EINVAL;
t = timespec64_to_ktime(ts);
if (cmd == FUTEX_WAIT)
if (cmd == FUTEX_WAIT || cmd == FUTEX_WAIT_MULTIPLE)
t = ktime_add_safe(ktime_get(), t);
tp = &t;
}
......@@ -3889,14 +4039,15 @@ SYSCALL_DEFINE6(futex_time32, u32 __user *, uaddr, int, op, u32, val,
if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
cmd == FUTEX_WAIT_BITSET ||
cmd == FUTEX_WAIT_REQUEUE_PI)) {
cmd == FUTEX_WAIT_REQUEUE_PI ||
cmd == FUTEX_WAIT_MULTIPLE)) {
if (get_old_timespec32(&ts, utime))
return -EFAULT;
if (!timespec64_valid(&ts))
return -EINVAL;
t = timespec64_to_ktime(ts);
if (cmd == FUTEX_WAIT)
if (cmd == FUTEX_WAIT || cmd == FUTEX_WAIT_MULTIPLE)
t = ktime_add_safe(ktime_get(), t);
tp = &t;
}
......
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