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    Subject[patch] change futex_wait() to hrtimers

    * Roland McGrath <roland@redhat.com> wrote:

    > I agree it should restart. But I don't think this is quite right in
    > the timeout case. It will increase the total maximum real time spent
    > arbitrarily by the amount of time elapsed in signal handlers. Other
    > restartable, timed calls have to convert to an absolute timeout for
    > the restart block (and convert back when doing the restart).

    i dont think we should try to do this. We should not and cannot do
    anything about all of the artifacts that comes with the use of relative
    timeouts and schedule_timeout().

    basically, using jiffies here (which schedule_timeout() does) is
    /fundamentally/ imprecise. If you get many interrupts, rounding errors
    sum up - and there's nothing we can do about it!

    the only correct approach is the use of hrtimers, and a patch exists for
    that - see below. This has been included in -rt for quite some time.

    so i'd suggest for Nick to add restart processing ontop of this patch:
    hrtimers use absolute timeouts and hence there are neither rounding nor
    signal processing delay artifacts.

    Ingo

    -------------->
    From: S�bastien Dugu� <sebastien.dugue@bull.net>
    Subject: [patch] change futex_wait() to hrtimers

    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 a timespec
    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 the timeout is zero then a regular schedule() is
    performed. Otherwise, an hrtimer is fired before schedule() is called.

    Signed-off-by: S�bastien Dugu� <sebastien.dugue@bull.net>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>

    include/linux/futex.h | 2 -
    kernel/futex.c | 61 +++++++++++++++++++++++++++++++++-----------------
    kernel/futex_compat.c | 11 +--------
    3 files changed, 44 insertions(+), 30 deletions(-)

    Index: linux/include/linux/futex.h
    ===================================================================
    --- linux.orig/include/linux/futex.h
    +++ linux/include/linux/futex.h
    @@ -94,7 +94,7 @@ 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, struct timespec *timeout,
    u32 __user *uaddr2, u32 val2, u32 val3);

    extern int
    Index: linux/kernel/futex.c
    ===================================================================
    --- linux.orig/kernel/futex.c
    +++ linux/kernel/futex.c
    @@ -49,6 +49,7 @@
    #include <linux/syscalls.h>
    #include <linux/signal.h>
    #include <asm/futex.h>
    +#include <linux/hrtimer.h>

    #include "rtmutex_common.h"

    @@ -1000,7 +1001,7 @@ static void unqueue_me_pi(struct futex_q
    drop_key_refs(&q->key);
    }

    -static int futex_wait(u32 __user *uaddr, u32 val, unsigned long time)
    +static int futex_wait(u32 __user *uaddr, u32 val, struct timespec *time)
    {
    struct task_struct *curr = current;
    DECLARE_WAITQUEUE(wait, curr);
    @@ -1008,6 +1009,8 @@ static int futex_wait(u32 __user *uaddr,
    struct futex_q q;
    u32 uval;
    int ret;
    + struct hrtimer_sleeper t;
    + int rem = 0;

    q.pi_state = NULL;
    retry:
    @@ -1085,8 +1088,33 @@ static int futex_wait(u32 __user *uaddr,
    * !list_empty() is safe here without any lock.
    * q.lock_ptr != 0 is not safe, because of ordering against wakeup.
    */
    - if (likely(!list_empty(&q.list)))
    - time = schedule_timeout(time);
    + if (likely(!list_empty(&q.list))) {
    + if (time->tv_sec == 0 && time->tv_nsec == 0)
    + schedule();
    + else {
    +
    + hrtimer_init(&t.timer, CLOCK_MONOTONIC,
    + HRTIMER_MODE_REL);
    + hrtimer_init_sleeper(&t, current);
    + t.timer.expires = timespec_to_ktime(*time);
    +
    + hrtimer_start(&t.timer, t.timer.expires,
    + HRTIMER_MODE_REL);
    +
    + /*
    + * 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();
    +
    + hrtimer_cancel(&t.timer);
    +
    + /* Flag if a timeout occured */
    + rem = (t.task == NULL);
    + }
    + }
    __set_current_state(TASK_RUNNING);

    /*
    @@ -1097,7 +1125,7 @@ static int futex_wait(u32 __user *uaddr,
    /* If we were woken (and unqueued), we succeeded, whatever. */
    if (!unqueue_me(&q))
    return 0;
    - if (time == 0)
    + if (rem)
    return -ETIMEDOUT;
    /*
    * We expect signal_pending(current), but another thread may
    @@ -1119,8 +1147,8 @@ static int futex_wait(u32 __user *uaddr,
    * 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, struct timespec *time,
    + int trylock)
    {
    struct hrtimer_sleeper timeout, *to = NULL;
    struct task_struct *curr = current;
    @@ -1132,11 +1160,11 @@ static int futex_lock_pi(u32 __user *uad
    if (refill_pi_state_cache())
    return -ENOMEM;

    - if (sec != MAX_SCHEDULE_TIMEOUT) {
    + if (time->tv_sec || time->tv_nsec) {
    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 = timespec_to_ktime(*time);
    }

    q.pi_state = NULL;
    @@ -1772,7 +1800,7 @@ 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, struct timespec *timeout,
    u32 __user *uaddr2, u32 val2, u32 val3)
    {
    int ret;
    @@ -1798,13 +1826,13 @@ 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;
    @@ -1817,8 +1845,7 @@ 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 t = {.tv_sec=0, .tv_nsec=0};
    u32 val2 = 0;

    if (utime && (op == FUTEX_WAIT || op == FUTEX_LOCK_PI)) {
    @@ -1826,12 +1853,6 @@ asmlinkage long sys_futex(u32 __user *ua
    return -EFAULT;
    if (!timespec_valid(&t))
    return -EINVAL;
    - if (op == FUTEX_WAIT)
    - timeout = timespec_to_jiffies(&t) + 1;
    - else {
    - timeout = t.tv_sec;
    - val2 = t.tv_nsec;
    - }
    }
    /*
    * requeue parameter in 'utime' if op == FUTEX_REQUEUE.
    @@ -1839,7 +1860,7 @@ 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, &t, uaddr2, val2, val3);
    }

    static int futexfs_get_sb(struct file_system_type *fs_type,
    Index: linux/kernel/futex_compat.c
    ===================================================================
    --- linux.orig/kernel/futex_compat.c
    +++ linux/kernel/futex_compat.c
    @@ -141,8 +141,7 @@ 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 t = {.tv_sec = 0, .tv_nsec = 0};
    int val2 = 0;

    if (utime && (op == FUTEX_WAIT || op == FUTEX_LOCK_PI)) {
    @@ -150,15 +149,9 @@ asmlinkage long compat_sys_futex(u32 __u
    return -EFAULT;
    if (!timespec_valid(&t))
    return -EINVAL;
    - if (op == FUTEX_WAIT)
    - timeout = timespec_to_jiffies(&t) + 1;
    - else {
    - timeout = t.tv_sec;
    - val2 = t.tv_nsec;
    - }
    }
    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, &t, uaddr2, val2, val3);
    }
    -
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