Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK_PI

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 03 Oct 2016 11:12:38 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> There's a number of 'interesting' problems with FUTEX_UNLOCK_PI, all
> caused by holding hb->lock while doing the rt_mutex_unlock()
> equivalient.
> 
> This patch doesn't attempt to fix any of the actual problems, but
> instead reworks the code to not hold hb->lock across the unlock,
> paving the way to actually fix the problems later.
> 
> The current reason we hold hb->lock over unlock is that it serializes
> against FUTEX_LOCK_PI and avoids new waiters from coming in, this then
> ensures the rt_mutex_next_owner() value is stable and can be written
> into the user-space futex value before doing the unlock. Such that the
> unlock will indeed end up at new_owner.
> 
> This patch recognises that holding rt_mutex::wait_lock results in the
> very same guarantee, no new waiters can come in while we hold that
> lock -- after all, waiters would need this lock to queue themselves.
> 
> It therefore restructures the code to keep rt_mutex::wait_lock held.
> 
> This (of course) is not entirely straight forward either, see the
> comment in rt_mutex_slowunlock(), doing the unlock itself might drop
> wait_lock, letting new waiters in. To cure this
> rt_mutex_futex_unlock() becomes a variant of rt_mutex_slowunlock()
> that return -EAGAIN instead. This ensures the FUTEX_UNLOCK_PI code
> aborts and restarts the entire operation.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/futex.c                  |   63 +++++++++++++++++--------------
>  kernel/locking/rtmutex.c        |   81 ++++++++++++++++++++++++++++++++++++----
>  kernel/locking/rtmutex_common.h |    4 -
>  3 files changed, 110 insertions(+), 38 deletions(-)
> 
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1294,24 +1294,21 @@ static void mark_wake_futex(struct wake_
>  static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter,
>  			 struct futex_hash_bucket *hb)
>  {
> -	struct task_struct *new_owner;
>  	struct futex_pi_state *pi_state = top_waiter->pi_state;
>  	u32 uninitialized_var(curval), newval;
> +	struct task_struct *new_owner;
>  	WAKE_Q(wake_q);
> -	bool deboost;
>  	int ret = 0;
>  
> -	if (!pi_state)
> -		return -EINVAL;
> +	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
>  
> +	WARN_ON_ONCE(!atomic_inc_not_zero(&pi_state->refcount));

Don't we have a rule where WARN_ON() and BUG_ON() should never have
"side effects"? That is, they should only check values, but their
contents should not update values.

hence have:

	ret = atomic_inc_not_zero(&pi_state->refcount);
	WARN_ON_ONCE(!ret);

>  	/*
> -	 * If current does not own the pi_state then the futex is
> -	 * inconsistent and user space fiddled with the futex value.
> +	 * Now that we hold wait_lock, no new waiters can happen on the
> +	 * rt_mutex and new owner is stable. Drop hb->lock.
>  	 */
> -	if (pi_state->owner != current)
> -		return -EINVAL;
> +	spin_unlock(&hb->lock);
>  

Also, as Sebastian has said before, I believe this breaks rt's migrate
disable code. As migrate disable and migrate_enable are a nop if
preemption is disabled, thus if you hold a raw_spin_lock across a
spin_unlock() when the migrate enable will be a nop, and the
migrate_disable() will never stop.

-- Steve

> -	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
>  	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
>  
>  	/*