On 10/20, Oleg Nesterov wrote:
>
> On 10/20, Oleg Nesterov wrote:
> >
> > Again, perhaps we will need to change the lifetime rules for task_struct
> > anyway, if we have more problems like this. But until then this looks like
> > an overkill to me. Plus rq_curr_if_not_put() looks too subtle, and it is
> > not generic.
>
> Yes... otoh, perhaps we can do something more generic? Something like
>
> struct task_struct *xxx(struct task_struct **ptask)
> {
> struct task_struct *task;
> void *sighand;
> retry:
> task = ACCESS_ONCE(*ptask);
> if (!task)
> return NULL;
>
> if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC)) {
> if (probe_kernel_read(&sighand, &task->sighand,
> sizeof(sighand)))
> goto retry;
> } else {
> sighand = task->sighand;
> }
>
> if (!sighand)
> return NULL;
> /*
> * Pairs with atomic_dec_and_test() in put_task_struct(task).
> * If we have read the freed/reused memory, we must see that
> * the pointer was updated.
> */
> smp_rmb();
> if (task != ACCESS_ONCE(*ptask))
> goto retry;
>
> return task;
> }
>
> task_numa_compare() can do cur = xxx(&rc->curr), but this helper can work
> with any "task_struct *" pointer assuming that somehow this pointer is
> cleared or changed before the final put_task_struct().
>
> What do you think? Peter?
And if we introduce this helper, it would better to check "sighand != NULL"
after "task != *ptask":
struct task_struct *xxx(struct task_struct **ptask)
{
struct task_struct *task;
struct sighand_struct *sighand;
retry:
task = ACCESS_ONCE(*ptask);
if (!task)
return task;
if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC)) {
if (probe_kernel_read(&sighand, &task->sighand,
sizeof(sighand)))
goto retry;
} else {
sighand = task->sighand;
}
/*
* Pairs with atomic_dec_and_test() in put_task_struct(task).
* If we have read the freed/reused memory, we must see that
* the pointer was updated.
*/
smp_rmb();
if (unlikely(task != ACCESS_ONCE(*ptask)))
goto retry;
/*
* release_task(task) was already called, potentially before
* the caller took rcu_read_lock() and in this case it can be
* freed before rcu_read_unlock().
*/
if (!sighand)
return NULL;
return task;
}
Of course, task_numa_compare() do not really need "retry", and task == NULL
is not possible. But this way the new helper can (probably) have more users,
and this just looks better imo.
Oleg.
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