* Oleg Nesterov <[email protected]> wrote:
> 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 think your worry is legitimate.
spin_lock() provides a full barrier on most platforms (certainly so on
x86). But ... ia64 might have it as a one-way barrier?
Ingo
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