Messages in this thread | | | Date | Tue, 23 May 2023 14:15:06 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression |
| |
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.
| |