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

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Message-ID: <20230531072449.GA25046@redhat.com>
Date:   Wed, 31 May 2023 09:25:11 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Mike Christie <michael.christie@...cle.com>, linux@...mhuis.info,
        nicolas.dichtel@...nd.com, axboe@...nel.dk, ebiederm@...ssion.com,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, mst@...hat.com,
        sgarzare@...hat.com, stefanha@...hat.com, brauner@...nel.org
Subject: Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps
 regression

On 05/31, Jason Wang wrote:
>
> 在 2023/5/23 20:15, Oleg Nesterov 写道:
> >
> >		/* 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.
>
> This should be fine since store is not speculated, so work->node->next needs
> to be loaded before VHOST_WORK_QUEUED is cleared to meet the loop condition.

I don't understand you. OK, to simplify, suppose we have 2 global vars

	void *PTR = something_non_null;
	unsigned long FLAGS = -1ul;

Now I think this code

	CPU_0				CPU_1

	void *ptr = PTR;		if (!test_and_set_bit(0, FLAGS))
	clear_bit(0, FLAGS);			PTR = NULL;
	BUG_ON(!ptr);

is racy and can hit the BUG_ON(!ptr).

I guess it is fine on x86, but in general you need smp_mb__before_atomic()
before clear_bit(), or clear_bit_unlock().

> >			__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 ?
>
> It is because the state were set to TASK_INTERRUPTIBLE in the beginning of
> the loop otherwise it might be side effect while executing work->fn().

Again, I don't understand you. So let me repeat: can work->fn() return with
current->_state != TASK_RUNNING ? If not (and I'd say it should not), you can
do __set_current_state(TASK_RUNNING) once, before llist_for_each_entry_safe().

> >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() ?
>
> It looks safe since:
>
> 1) vhost hold refcnt of the mm
> 2) release will sync with the worker

Well, that's not what I asked... nevermind, please forget.

Thanks.

Oleg.