Re: [PATCH] futex: Handle transient "ownerless" rtmutex state correctly

[Mike Galbraith <efault@gmx.de>]

On Wed, 2020-11-04 at 16:12 +0100, Thomas Gleixner wrote:
> From: Mike Galbraith <efault@gmx.de>

Hrmph.  How about a suggested-by, or just take it.  That dinky diag hit
the mark (not _entirely_ by accident, but..;) is irrelevant, you did
all of the work to make it a patch.

	-Mike

> Gratian managed to trigger the BUG_ON(!newowner) in fixup_pi_state_owner().
> This is one possible chain of events leading to this:
>
> Task Prio       Operation
> T1   120	lock(F)
> T2   120	lock(F)   -> blocks (top waiter)
> T3   50 (RT)	lock(F)   -> boosts T3 and blocks (new top waiter)
> XX   		timeout/  -> wakes T2
> 		signal
> T1   50		unlock(F) -> wakes T3 (rtmutex->owner == NULL, waiter bit is set)
> T2   120	cleanup   -> try_to_take_mutex() fails because T3 is the top waiter
>      			     and the lower priority T2 cannot steal the lock.
>      			  -> fixup_pi_state_owner() sees newowner == NULL -> BUG_ON()
>
> The comment states that this is invalid and rt_mutex_real_owner() must
> return a non NULL owner when the trylock failed, but in case of a queued
> and woken up waiter rt_mutex_real_owner() == NULL is a valid transient
> state. The higher priority waiter has simply not yet managed to take over
> the rtmutex.
>
> The BUG_ON() is therefore wrong and this is just another retry condition in
> fixup_pi_state_owner().
>
> Drop the locks, so that T3 can make progress, and then try the fixup again.
>
> Gratian provided a great analysis, traces and a reproducer. The analysis is
> to the point, but it confused the hell out of that tglx dude who had to
> page in all the futex horrors again. Condensed version is above.
>
> [ tglx: Wrote comment and changelog ]
>
> Fixes: c1e2f0eaf015 ("futex: Avoid violating the 10th rule of futex")
> Reported-by: Gratian Crisan <gratian.crisan@ni.com>
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: stable@vger.kernel.org
> Link: https://lore.kernel.org/r/87a6w6x7bb.fsf@ni.com
> ---
>  kernel/futex.c |   16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
>
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -2380,10 +2380,22 @@ static int fixup_pi_state_owner(u32 __us
>  		}
>
>  		/*
> -		 * Since we just failed the trylock; there must be an owner.
> +		 * The trylock just failed, so either there is an owner or
> +		 * there is a higher priority waiter than this one.
>  		 */
>  		newowner = rt_mutex_owner(&pi_state->pi_mutex);
> -		BUG_ON(!newowner);
> +		/*
> +		 * If the higher priority waiter has not yet taken over the
> +		 * rtmutex then newowner is NULL. We can't return here with
> +		 * that state because it's inconsistent vs. the user space
> +		 * state. So drop the locks and try again. It's a valid
> +		 * situation and not any different from the other retry
> +		 * conditions.
> +		 */
> +		if (unlikely(!newowner)) {
> +			ret = -EAGAIN;
> +			goto handle_err;
> +		}
>  	} else {
>  		WARN_ON_ONCE(argowner != current);
>  		if (oldowner == current) {