On Wed, 2020-11-04 at 16:12 +0100, Thomas Gleixner wrote:
> From: Mike Galbraith <efault@xxxxxx>
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@xxxxxx>
> Signed-off-by: Mike Galbraith <efault@xxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/r/87a6w6x7bb.fsf@xxxxxx
> ---
> 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) {
