From: Peter Oskolkov <[email protected]> See the previous patch in the patchset, which introduced FUTEX_SWAP op in futex.h, for a detailed description of the use cases and future directions.
This patch implements FUTEX_SWAP as a simple wake+wait. The next patch will improve this by migrating the wakee to the waker's (= waiter's) CPU. Tested: see patch 4 in this patchset. Signed-off-by: Peter Oskolkov <[email protected]> --- kernel/futex.c | 87 +++++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 72 insertions(+), 15 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index 4616d4ad609d..a81d62a16e72 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1574,16 +1574,16 @@ double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2) } /* - * Wake up waiters matching bitset queued on this futex (uaddr). + * Prepare wake queue matching bitset queued on this futex (uaddr). */ static int -futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset) +prepare_wake_q(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset, + struct wake_q_head *wake_q) { struct futex_hash_bucket *hb; struct futex_q *this, *next; union futex_key key = FUTEX_KEY_INIT; int ret; - DEFINE_WAKE_Q(wake_q); if (!bitset) return -EINVAL; @@ -1611,13 +1611,26 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset) if (!(this->bitset & bitset)) continue; - mark_wake_futex(&wake_q, this); + mark_wake_futex(wake_q, this); if (++ret >= nr_wake) break; } } spin_unlock(&hb->lock); + return ret; +} + +/* + * Wake up waiters matching bitset queued on this futex (uaddr). + */ +static int +futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset) +{ + int ret; + DEFINE_WAKE_Q(wake_q); + + ret = prepare_wake_q(uaddr, flags, nr_wake, bitset, &wake_q); wake_up_q(&wake_q); return ret; } @@ -2557,9 +2570,12 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked) * @hb: the futex hash bucket, must be locked by the caller * @q: the futex_q to queue up on * @timeout: the prepared hrtimer_sleeper, or null for no timeout + * @next: if present, wake next and hint to the scheduler that we'd + * prefer to execute it locally. */ static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q, - struct hrtimer_sleeper *timeout) + struct hrtimer_sleeper *timeout, + struct task_struct *next) { /* * The task state is guaranteed to be set before another task can @@ -2584,10 +2600,26 @@ static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q, * flagged for rescheduling. Only call schedule if there * is no timeout, or if it has yet to expire. */ - if (!timeout || timeout->task) + if (!timeout || timeout->task) { + if (next) { + /* + * wake_up_process() below will be + * replaced in the next patch with + * wake_up_swap(). + */ + wake_up_process(next); + put_task_struct(next); + next = NULL; + } freezable_schedule(); + } } __set_current_state(TASK_RUNNING); + + if (next) { + wake_up_process(next); + put_task_struct(next); + } } /** @@ -2663,7 +2695,7 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags, } static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val, - ktime_t *abs_time, u32 bitset) + ktime_t *abs_time, u32 bitset, struct task_struct *next) { struct hrtimer_sleeper timeout, *to; struct restart_block *restart; @@ -2687,7 +2719,8 @@ static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val, goto out; /* queue_me and wait for wakeup, timeout, or a signal. */ - futex_wait_queue_me(hb, &q, to); + futex_wait_queue_me(hb, &q, to, next); + next = NULL; /* If we were woken (and unqueued), we succeeded, whatever. */ ret = 0; @@ -2720,6 +2753,10 @@ static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val, ret = -ERESTART_RESTARTBLOCK; out: + if (next) { + wake_up_process(next); + put_task_struct(next); + } if (to) { hrtimer_cancel(&to->timer); destroy_hrtimer_on_stack(&to->timer); @@ -2727,7 +2764,6 @@ static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val, return ret; } - static long futex_wait_restart(struct restart_block *restart) { u32 __user *uaddr = restart->futex.uaddr; @@ -2739,10 +2775,29 @@ static long futex_wait_restart(struct restart_block *restart) } restart->fn = do_no_restart_syscall; - return (long)futex_wait(uaddr, restart->futex.flags, - restart->futex.val, tp, restart->futex.bitset); + return (long)futex_wait(uaddr, restart->futex.flags, restart->futex.val, + tp, restart->futex.bitset, NULL); } +static int futex_swap(u32 __user *uaddr, unsigned int flags, u32 val, + ktime_t *abs_time, u32 __user *uaddr2) +{ + u32 bitset = FUTEX_BITSET_MATCH_ANY; + struct task_struct *next = NULL; + DEFINE_WAKE_Q(wake_q); + int ret; + + ret = prepare_wake_q(uaddr2, flags, 1, bitset, &wake_q); + if (ret < 0) + return ret; + if (!wake_q_empty(&wake_q)) { + /* At most one wakee can be present. Pull it out. */ + next = container_of(wake_q.first, struct task_struct, wake_q); + next->wake_q.next = NULL; + } + + return futex_wait(uaddr, flags, val, abs_time, bitset, next); +} /* * Userspace tried a 0 -> TID atomic transition of the futex value @@ -3221,7 +3276,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, } /* Queue the futex_q, drop the hb lock, wait for wakeup. */ - futex_wait_queue_me(hb, &q, to); + futex_wait_queue_me(hb, &q, to, NULL); spin_lock(&hb->lock); ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to); @@ -3746,7 +3801,7 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, val3 = FUTEX_BITSET_MATCH_ANY; /* fall through */ case FUTEX_WAIT_BITSET: - return futex_wait(uaddr, flags, val, timeout, val3); + return futex_wait(uaddr, flags, val, timeout, val3, NULL); case FUTEX_WAKE: val3 = FUTEX_BITSET_MATCH_ANY; /* fall through */ @@ -3770,6 +3825,8 @@ 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_SWAP: + return futex_swap(uaddr, flags, val, timeout, uaddr2); } return -ENOSYS; } @@ -3786,7 +3843,7 @@ 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_SWAP)) { if (unlikely(should_fail_futex(!(op & FUTEX_PRIVATE_FLAG)))) return -EFAULT; if (get_timespec64(&ts, utime)) @@ -3795,7 +3852,7 @@ 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_SWAP) t = ktime_add_safe(ktime_get(), t); tp = &t; } -- 2.25.1
