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SubjectRe: [patch 08/12] futex: Sanitize exit state handling
On Wed, Nov 06, 2019 at 10:55:42PM +0100, Thomas Gleixner wrote:
> Instead of having a smp_mb() and an empty lock/unlock of task::pi_lock move
> the state setting into to the lock section.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> kernel/futex.c | 17 ++++++++++-------
> 1 file changed, 10 insertions(+), 7 deletions(-)
>
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -3703,16 +3703,19 @@ void futex_exit_done(struct task_struct
>
> void futex_exit_release(struct task_struct *tsk)
> {
> - tsk->futex_state = FUTEX_STATE_EXITING;
> - /*
> - * Ensure that all new tsk->pi_lock acquisitions must observe
> - * FUTEX_STATE_EXITING. Serializes against attach_to_pi_owner().
> - */
> - smp_mb();
> /*
> - * Ensure that we must observe the pi_state in exit_pi_state_list().
> + * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
> + *
> + * This ensures that all subsequent checks of tsk->futex_state in
> + * attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
> + * tsk->pi_lock held.
> + *
> + * It guarantees also that a pi_state which was queued right before
> + * the state change under tsk->pi_lock by a concurrent waiter must
> + * be observed in exit_pi_state_list().
> */
> raw_spin_lock_irq(&tsk->pi_lock);
> + tsk->futex_state = FUTEX_STATE_EXITING;
> raw_spin_unlock_irq(&tsk->pi_lock);

Right, much saner.

So this used to be:

exit_signals(tsk) /* sets PF_EXITING */
smp_mb();
raw_spin_unlock_wait(&tsk->pi_lock);

Which is in fact (possibly) faster than the new sane code, since
unlock_wait() only has to wait for any current lock holder to complete.

However due to terrible semantics and implementation issues we got rid
of *spin_unlock_wait() and well.. lets all forget this :-)

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