Re: [PATCH -v5 14/14] futex: futex_unlock_pi() determinism

[Thomas Gleixner]

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) <peterz@xxxxxxxxxxxxx>
> ---
>  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