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SubjectRe: [PATCH] pi-futex: set PF_EXITING without taking ->pi_lock
On 07/21, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@tv-sign.ru> wrote:
>
> > static inline void ccids_read_lock(void)
> > {
> > atomic_inc(&ccids_lockct);
> > spin_unlock_wait(&ccids_lock);
> > }
> >
> > This looks racy, in theory atomic_inc() and spin_unlock_wait() could
> > be re-ordered. However, in this particular case we have an "optimized"
> > smp_mb_after_atomic_inc(), perhaps it is good that the caller can
> > choose the "right" barrier by hand.
>
> _all_ default locking and atomic APIs should be barrier-safe i believe.
> (and that includes atomic_inc() too) Most people dont have barriers on
> their mind when their code. _If_ someone is barrier-conscious then we
> should have barrier-less APIs too for that purpose of squeezing the last
> half cycle out of the code, but it should be a non-default choice. The
> reason: nobody notices an unnecessary barrier, but a missing barrier can
> be nasty.

Personally, I agree (but I am not sure the idea to make atomic_inc()
barrier-safe would be very popular).

Question: should we make spinlock_t barrier-safe?

Suppose that the task "p" does

current->state = TASK_INTERRUPIBLE;
mb();

if (CONDITION)
break;
schedule();

and another CPU does

CONDITION = 1;
try_to_wake_up(p);


This is commonly used, but not correct _in theory_. If wake_up() happens
when p->array != NULL, we have

CONDITION = 1; // [1]
spin_lock(rq->lock);
task->state = TASK_RUNNING; // [2]
and we can miss an event. Because in theory [1] may leak into the critical
section, and could be re-ordered with [2].

Another problem is that try_to_wake_up() first checks task->state and does
nothing if it is TASK_RUNNING, so we need a full mb(), not just wmb().

Should we change spin_lock(), or introduce smp_mb_before_spinlock(), or I
missed something?

NOTE: I do not pretend to know what kind of barrier spin_lock() provides
in practice, but according to the documentation lock() is only a one-way
barrier.

(I am glad I have an opportunity to raise this issue again :)

Oleg.

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