From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> commit 4a8e991b91aca9e20705d434677ac013974e0e30 upstream. 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@xxxxxxxxxxxxx> Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Link: https://lkml.kernel.org/r/20191106224556.645603214@xxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/futex.c | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-) --- a/kernel/futex.c +++ b/kernel/futex.c @@ -3726,16 +3726,19 @@ void futex_exit_recursive(struct task_st 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); futex_exec_release(tsk);
