On Sat, 4 Mar 2017, Peter Zijlstra wrote:
> The problem with returning -EAGAIN when the waiter state mismatches is
> that it becomes very hard to proof a bounded execution time on the
> operation. And seeing that this is a RT operation, this is somewhat
> important.
>
> While in practise it will be very unlikely to ever really take more
> than one or two rounds, proving so becomes rather hard.
Oh no. Assume the following:
T1 and T2 are both pinned to CPU0. prio(T2) > prio(T1)
CPU0
T1
lock_pi()
queue_me() <- Waiter is visible
preemption
T2
unlock_pi()
loops with -EAGAIN forever
> Now that modifying wait_list is done while holding both hb->lock and
> wait_lock, we can avoid the scenario entirely if we acquire wait_lock
> while still holding hb-lock. Doing a hand-over, without leaving a
> hole.
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> ---
> kernel/futex.c | 26 ++++++++++++--------------
> 1 file changed, 12 insertions(+), 14 deletions(-)
>
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1391,16 +1391,11 @@ static int wake_futex_pi(u32 __user *uad
> DEFINE_WAKE_Q(wake_q);
> int ret = 0;
>
> - raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
> new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
> - if (!new_owner) {
> + if (WARN_ON_ONCE(!new_owner)) {
> /*
> - * Since we held neither hb->lock nor wait_lock when coming
> - * into this function, we could have raced with futex_lock_pi()
> - * such that it will have removed the waiter that brought us
> - * here.
> - *
> - * In this case, retry the entire operation.
> + * Should be impossible now... but if weirdness happens,
'now...' is not very useful 6 month from NOW :)
> + * returning -EAGAIN is safe and correct.
> */
> ret = -EAGAIN;
> goto out_unlock;
> @@ -2770,15 +2765,18 @@ static int futex_unlock_pi(u32 __user *u
> if (pi_state->owner != current)
> goto out_unlock;
>
> + get_pi_state(pi_state);
> /*
> - * Grab a reference on the pi_state and drop hb->lock.
> + * Since modifying the wait_list is done while holding both
> + * hb->lock and wait_lock, holding either is sufficient to
> + * observe it.
> *
> - * The reference ensures pi_state lives, dropping the hb->lock
> - * is tricky.. wake_futex_pi() will take rt_mutex::wait_lock to
> - * close the races against futex_lock_pi(), but in case of
> - * _any_ fail we'll abort and retry the whole deal.
> + * By taking wait_lock while still holding hb->lock, we ensure
> + * there is no point where we hold neither; and therefore
> + * wake_futex_pi() must observe a state consistent with what we
> + * observed.
> */
> - get_pi_state(pi_state);
> + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
> spin_unlock(&hb->lock);
Other than that, this pretty good.
Thanks,
tglx
