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Subject[PATCH 2.6.21-rc4-mm1 2/4] Make futex_wait() use an hrtimer for timeout
This patch modifies futex_wait() to use an hrtimer + schedule() in place of
schedule_timeout().

schedule_timeout() is tick based, therefore the timeout granularity is
the tick (1 ms, 4 ms or 10 ms depending on HZ). By using a high resolution
timer for timeout wakeup, we can attain a much finer timeout granularity
(in the microsecond range). This parallels what is already done for
futex_lock_pi().

The timeout passed to the syscall is no longer converted to jiffies
and is therefore passed to do_futex() and futex_wait() as an absolute
ktime_t therefore keeping nanosecond resolution.

Also this removes the need to pass the nanoseconds timeout part to
futex_lock_pi() in val2.

In futex_wait(), if there is no timeout then a regular schedule() is
performed. Otherwise, an hrtimer is fired before schedule() is called.

Signed-off-by: Sebastien Dugue <sebastien.dugue@bull.net>
Signed-off-by: Pierre Peiffer <pierre.peiffer@bull.net>

---
include/linux/futex.h | 3 +
kernel/futex.c | 85 ++++++++++++++++++++++++--------------------------
kernel/futex_compat.c | 17 ++++------
3 files changed, 51 insertions(+), 54 deletions(-)
Index: b/kernel/futex.c
===================================================================
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1001,16 +1001,16 @@ static void unqueue_me_pi(struct futex_q
}

static long futex_wait_restart(struct restart_block *restart);
-static int futex_wait_abstime(u32 __user *uaddr, u32 val,
- int timed, unsigned long abs_time)
+static int futex_wait(u32 __user *uaddr, u32 val, ktime_t *abs_time)
{
struct task_struct *curr = current;
DECLARE_WAITQUEUE(wait, curr);
struct futex_hash_bucket *hb;
struct futex_q q;
- unsigned long time_left = 0;
u32 uval;
int ret;
+ struct hrtimer_sleeper t;
+ int rem = 0;

q.pi_state = NULL;
retry:
@@ -1088,20 +1088,29 @@ static int futex_wait_abstime(u32 __user
* !plist_node_empty() is safe here without any lock.
* q.lock_ptr != 0 is not safe, because of ordering against wakeup.
*/
- time_left = 0;
if (likely(!plist_node_empty(&q.list))) {
- unsigned long rel_time;
+ if (!abs_time)
+ schedule();
+ else {
+ hrtimer_init(&t.timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
+ hrtimer_init_sleeper(&t, current);
+ t.timer.expires = *abs_time;
+
+ hrtimer_start(&t.timer, t.timer.expires, HRTIMER_MODE_ABS);
+
+ /*
+ * the timer could have already expired, in which
+ * case current would be flagged for rescheduling.
+ * Don't bother calling schedule.
+ */
+ if (likely(t.task))
+ schedule();

- if (timed) {
- unsigned long now = jiffies;
- if (time_after(now, abs_time))
- rel_time = 0;
- else
- rel_time = abs_time - now;
- } else
- rel_time = MAX_SCHEDULE_TIMEOUT;
+ hrtimer_cancel(&t.timer);

- time_left = schedule_timeout(rel_time);
+ /* Flag if a timeout occured */
+ rem = (t.task == NULL);
+ }
}
__set_current_state(TASK_RUNNING);

@@ -1113,14 +1122,14 @@ static int futex_wait_abstime(u32 __user
/* If we were woken (and unqueued), we succeeded, whatever. */
if (!unqueue_me(&q))
return 0;
- if (time_left == 0)
+ if (rem)
return -ETIMEDOUT;

/*
* We expect signal_pending(current), but another thread may
* have handled it for us already.
*/
- if (time_left == MAX_SCHEDULE_TIMEOUT)
+ if (!abs_time)
return -ERESTARTSYS;
else {
struct restart_block *restart;
@@ -1128,8 +1137,7 @@ static int futex_wait_abstime(u32 __user
restart->fn = futex_wait_restart;
restart->arg0 = (unsigned long)uaddr;
restart->arg1 = (unsigned long)val;
- restart->arg2 = (unsigned long)timed;
- restart->arg3 = abs_time;
+ restart->arg2 = (unsigned long)abs_time;
return -ERESTART_RESTARTBLOCK;
}

@@ -1141,21 +1149,15 @@ static int futex_wait_abstime(u32 __user
return ret;
}

-static int futex_wait(u32 __user *uaddr, u32 val, unsigned long rel_time)
-{
- int timed = (rel_time != MAX_SCHEDULE_TIMEOUT);
- return futex_wait_abstime(uaddr, val, timed, jiffies+rel_time);
-}

static long futex_wait_restart(struct restart_block *restart)
{
u32 __user *uaddr = (u32 __user *)restart->arg0;
u32 val = (u32)restart->arg1;
- int timed = (int)restart->arg2;
- unsigned long abs_time = restart->arg3;
+ ktime_t *abs_time = (ktime_t *)restart->arg2;

restart->fn = do_no_restart_syscall;
- return (long)futex_wait_abstime(uaddr, val, timed, abs_time);
+ return (long)futex_wait(uaddr, val, abs_time);
}


@@ -1165,8 +1167,8 @@ static long futex_wait_restart(struct re
* if there are waiters then it will block, it does PI, etc. (Due to
* races the kernel might see a 0 value of the futex too.)
*/
-static int futex_lock_pi(u32 __user *uaddr, int detect, unsigned long sec,
- long nsec, int trylock)
+static int futex_lock_pi(u32 __user *uaddr, int detect, ktime_t *time,
+ int trylock)
{
struct hrtimer_sleeper timeout, *to = NULL;
struct task_struct *curr = current;
@@ -1178,11 +1180,11 @@ static int futex_lock_pi(u32 __user *uad
if (refill_pi_state_cache())
return -ENOMEM;

- if (sec != MAX_SCHEDULE_TIMEOUT) {
+ if (time) {
to = &timeout;
hrtimer_init(&to->timer, CLOCK_REALTIME, HRTIMER_MODE_ABS);
hrtimer_init_sleeper(to, current);
- to->timer.expires = ktime_set(sec, nsec);
+ to->timer.expires = *time;
}

q.pi_state = NULL;
@@ -1818,7 +1820,7 @@ void exit_robust_list(struct task_struct
}
}

-long do_futex(u32 __user *uaddr, int op, u32 val, unsigned long timeout,
+long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
u32 __user *uaddr2, u32 val2, u32 val3)
{
int ret;
@@ -1844,13 +1846,13 @@ long do_futex(u32 __user *uaddr, int op,
ret = futex_wake_op(uaddr, uaddr2, val, val2, val3);
break;
case FUTEX_LOCK_PI:
- ret = futex_lock_pi(uaddr, val, timeout, val2, 0);
+ ret = futex_lock_pi(uaddr, val, timeout, 0);
break;
case FUTEX_UNLOCK_PI:
ret = futex_unlock_pi(uaddr);
break;
case FUTEX_TRYLOCK_PI:
- ret = futex_lock_pi(uaddr, 0, timeout, val2, 1);
+ ret = futex_lock_pi(uaddr, 0, timeout, 1);
break;
default:
ret = -ENOSYS;
@@ -1863,21 +1865,18 @@ asmlinkage long sys_futex(u32 __user *ua
struct timespec __user *utime, u32 __user *uaddr2,
u32 val3)
{
- struct timespec t;
- unsigned long timeout = MAX_SCHEDULE_TIMEOUT;
+ struct timespec ts;
+ ktime_t t, *tp = NULL;
u32 val2 = 0;

if (utime && (op == FUTEX_WAIT || op == FUTEX_LOCK_PI)) {
- if (copy_from_user(&t, utime, sizeof(t)) != 0)
+ if (copy_from_user(&ts, utime, sizeof(ts)) != 0)
return -EFAULT;
- if (!timespec_valid(&t))
+ if (!timespec_valid(&ts))
return -EINVAL;
if (op == FUTEX_WAIT)
- timeout = timespec_to_jiffies(&t) + 1;
- else {
- timeout = t.tv_sec;
- val2 = t.tv_nsec;
- }
+ t = ktime_add(ktime_get(), timespec_to_ktime(ts));
+ tp = &t;
}
/*
* requeue parameter in 'utime' if op == FUTEX_REQUEUE.
@@ -1885,7 +1884,7 @@ asmlinkage long sys_futex(u32 __user *ua
if (op == FUTEX_REQUEUE || op == FUTEX_CMP_REQUEUE)
val2 = (u32) (unsigned long) utime;

- return do_futex(uaddr, op, val, timeout, uaddr2, val2, val3);
+ return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
}

static int futexfs_get_sb(struct file_system_type *fs_type,
Index: b/include/linux/futex.h
===================================================================
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -1,6 +1,7 @@
#ifndef _LINUX_FUTEX_H
#define _LINUX_FUTEX_H

+#include <linux/ktime.h>
#include <linux/sched.h>

/* Second argument to futex syscall */
@@ -94,7 +95,7 @@ struct robust_list_head {
#define ROBUST_LIST_LIMIT 2048

#ifdef __KERNEL__
-long do_futex(u32 __user *uaddr, int op, u32 val, unsigned long timeout,
+long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
u32 __user *uaddr2, u32 val2, u32 val3);

extern int
Index: b/kernel/futex_compat.c
===================================================================
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -141,24 +141,21 @@ asmlinkage long compat_sys_futex(u32 __u
struct compat_timespec __user *utime, u32 __user *uaddr2,
u32 val3)
{
- struct timespec t;
- unsigned long timeout = MAX_SCHEDULE_TIMEOUT;
+ struct timespec ts;
+ ktime_t t, *tp = NULL;
int val2 = 0;

if (utime && (op == FUTEX_WAIT || op == FUTEX_LOCK_PI)) {
- if (get_compat_timespec(&t, utime))
+ if (get_compat_timespec(&ts, utime))
return -EFAULT;
- if (!timespec_valid(&t))
+ if (!timespec_valid(&ts))
return -EINVAL;
if (op == FUTEX_WAIT)
- timeout = timespec_to_jiffies(&t) + 1;
- else {
- timeout = t.tv_sec;
- val2 = t.tv_nsec;
- }
+ t = ktime_add(ktime_get(), timespec_to_ktime(ts));
+ tp = &t;
}
if (op == FUTEX_REQUEUE || op == FUTEX_CMP_REQUEUE)
val2 = (int) (unsigned long) utime;

- return do_futex(uaddr, op, val, timeout, uaddr2, val2, val3);
+ return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
}
--
Pierre Peiffer
-
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