linux-kernel - Re: [PATCH -v5 09/14] futex: Rework inconsistent rt_mutex/futex

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Date:   Tue, 7 Mar 2017 14:26:37 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     mingo@...nel.org, juri.lelli@....com, rostedt@...dmis.org,
        xlpang@...hat.com, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com, dvhart@...radead.org
Subject: Re: [PATCH -v5 09/14] futex: Rework inconsistent rt_mutex/futex_q
 state

On Sat, 4 Mar 2017, Peter Zijlstra wrote:
> @@ -1402,12 +1402,18 @@ static int wake_futex_pi(u32 __user *uad
>  	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
>  
>  	/*
> -	 * It is possible that the next waiter (the one that brought
> -	 * top_waiter owner to the kernel) timed out and is no longer
> -	 * waiting on the lock.
> +	 * When we interleave with futex_lock_pi() where it does
> +	 * rt_mutex_timed_futex_lock(), we might observe @this futex_q waiter,
> +	 * but the rt_mutex's wait_list can be empty (either still, or again,
> +	 * depending on which side we land).
> +	 *
> +	 * When this happens, give up our locks and try again, giving the
> +	 * futex_lock_pi() instance time to complete and unqueue_me().

  time to complete, either by waiting on the rtmutex or removing itself
  from the futex queue.

Or something like that.

Thanks,

	tglx