linux-kernel - Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

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Date:   Tue, 23 May 2023 10:57:04 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Mike Christie <michael.christie@...cle.com>, linux@...mhuis.info,
        nicolas.dichtel@...nd.com, axboe@...nel.dk,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, mst@...hat.com,
        sgarzare@...hat.com, jasowang@...hat.com, stefanha@...hat.com,
        brauner@...nel.org
Subject: Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps
 regression

Oleg Nesterov <oleg@...hat.com> writes:

> 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.

Very much agreed.  It is one thing to add a patch to move do_exit
out of get_signal.  It is another to keep calling get_signal after
that.  Nothing tests that case, and so we get some weird behaviors.


>> 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;
> 		}

I want to point out that we need to consider not just SIGKILL, but
SIGABRT that causes a coredump, as well as the process peforming
an ordinary exit(2).  All of which will cause get_signal to return
SIGKILL in this context.

>
> -------------------------------------------------------------------------------
> But let me ask a couple of questions.

I share most of these 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?

In a conversation long ago I remember hearing that vhost does not
support file descriptor passing.  Which means all of the file
descriptors should be in the same process.

Looking at the vhost code what I am seeing happening is that the
vhost_worker persists until vhost_dev_cleanup is called from
one of the vhost_???_release() functions.  The release functions
are only called after the last flush function completes.  See __fput
if you want to trace the details.


On one hand this all seems reasonable.  On the other hand I am not
seeing the code that prevents file descriptor passing.


It is probably not the worst thing in the world, but what this means
is now if you pass a copy of the vhost file descriptor to another
process the vhost_worker will persist, and thus the process will persist
until that copy of the file descriptor is closed.

Eric