Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

Oleg Nesterov
2015-11-11T18:57:51Z

He Boqun,

Let me first state that I can't answer authoritatively when it comes to
barriers. That said,

On 11/11, Boqun Feng wrote:
>
> But still, there is one suspicious use of smp_mb() in do_exit():
>
> /*
> * The setting of TASK_RUNNING by try_to_wake_up() may be delayed
> * when the following two conditions become true.
> * - There is race condition of mmap_sem (It is acquired by
> * exit_mm()), and
> * - SMI occurs before setting TASK_RUNINNG.
> * (or hypervisor of virtual machine switches to other guest)
> * As a result, we may become TASK_RUNNING after becoming TASK_DEAD
> *
> * To avoid it, we have to wait for releasing tsk->pi_lock which
> * is held by try_to_wake_up()
> */
> smp_mb();
> raw_spin_unlock_wait(&tsk->pi_lock);
>
> /* causes final put_task_struct in finish_task_switch(). */
> tsk->state = TASK_DEAD;
> tsk->flags |= PF_NOFREEZE; /* tell freezer to ignore us */
> schedule();
>
> Seems like smp_mb() doesn't need here?

Please see my reply to peterz's email.

AFAICS, we need the barries on both sides. But, since we only need to
STORE into tsk->state after unlock_wait(), we can rely on the control
dependency and avoid the 2nd mb().

Oleg.