Hey Oleg,
For all these questions below let me get back to you by tomorrow.
I need to confirm if something would be considered a regression by
the core vhost developers.
On 5/23/23 7:15 AM, Oleg Nesterov wrote:
> On 05/22, Oleg Nesterov wrote:
>>
>> Right now I think that "int dead" should die,
>
> No, probably we shouldn't call get_signal() if we have already dequeued SIGKILL.
>
>> but let me think tomorrow.
>
> May be something like this... I don't like it but I can't suggest anything better
> right now.
>
> bool killed = false;
>
> for (;;) {
> ...
>
> node = llist_del_all(&worker->work_list);
> if (!node) {
> schedule();
> /*
> * When we get a SIGKILL our release function will
> * be called. That will stop new IOs from being queued
> * and check for outstanding cmd responses. It will then
> * call vhost_task_stop to tell us to return and exit.
> */
> if (signal_pending(current)) {
> struct ksignal ksig;
>
> if (!killed)
> killed = get_signal(&ksig);
>
> clear_thread_flag(TIF_SIGPENDING);
> }
>
> continue;
> }
>
> -------------------------------------------------------------------------------
> But let me ask a couple of questions. Let's forget this patch, let's look at the
> current code:
>
> node = llist_del_all(&worker->work_list);
> if (!node)
> schedule();
>
> node = llist_reverse_order(node);
> ... process works ...
>
> To me this looks a bit confusing. Shouldn't we do
>
> if (!node) {
> schedule();
> continue;
> }
>
> just to make the code a bit more clear? If node == NULL then
> llist_reverse_order() and llist_for_each_entry_safe() will do nothing.
> But this is minor.
>
>
>
> /* make sure flag is seen after deletion */
> smp_wmb();
> llist_for_each_entry_safe(work, work_next, node, node) {
> clear_bit(VHOST_WORK_QUEUED, &work->flags);
>
> I am not sure about smp_wmb + clear_bit. Once we clear VHOST_WORK_QUEUED,
> vhost_work_queue() can add this work again and change work->node->next.
>
> That is why we use _safe, but we need to ensure that llist_for_each_safe()
> completes LOAD(work->node->next) before VHOST_WORK_QUEUED is cleared.
>
> So it seems that smp_wmb() can't help and should be removed, instead we need
>
> llist_for_each_entry_safe(...) {
> smp_mb__before_atomic();
> clear_bit(VHOST_WORK_QUEUED, &work->flags);
>
> Also, if the work->fn pointer is not stable, we should read it before
> smp_mb__before_atomic() as well.
>
> No?
>
>
> __set_current_state(TASK_RUNNING);
>
> Why do we set TASK_RUNNING inside the loop? Does this mean that work->fn()
> can return with current->state != RUNNING ?
>
>
> work->fn(work);
>
> Now the main question. Whatever we do, SIGKILL/SIGSTOP/etc can come right
> before we call work->fn(). Is it "safe" to run this callback with
> signal_pending() or fatal_signal_pending() ?
>
>
> Finally. I never looked into drivers/vhost/ before so I don't understand
> this code at all, but let me ask anyway... Can we change vhost_dev_flush()
> to run the pending callbacks rather than wait for vhost_worker() ?
> I guess we can't, ->mm won't be correct, but can you confirm?
>
> Oleg.
>
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