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?
Oleg.
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