linux-kernel - Re: [PATCH 1/1] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

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Date:   Fri, 2 Jun 2023 21:22:55 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Mike Christie <michael.christie@...cle.com>
Cc:     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, jasowang@...hat.com, stefanha@...hat.com,
        brauner@...nel.org
Subject: Re: [PATCH 1/1] fork, vhost: Use CLONE_THREAD to fix freezer/ps
 regression

Hi Mike,

sorry, but somehow I can't understand this patch...

I'll try to read it with a fresh head on Weekend, but for example,

On 06/01, Mike Christie wrote:
>
>  static int vhost_task_fn(void *data)
>  {
>  	struct vhost_task *vtsk = data;
> -	int ret;
> +	bool dead = false;
> +
> +	for (;;) {
> +		bool did_work;
> +
> +		/* mb paired w/ vhost_task_stop */
> +		if (test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags))
> +			break;
> +
> +		if (!dead && signal_pending(current)) {
> +			struct ksignal ksig;
> +			/*
> +			 * Calling get_signal will block in SIGSTOP,
> +			 * or clear fatal_signal_pending, but remember
> +			 * what was set.
> +			 *
> +			 * This thread won't actually exit until all
> +			 * of the file descriptors are closed, and
> +			 * the release function is called.
> +			 */
> +			dead = get_signal(&ksig);
> +			if (dead)
> +				clear_thread_flag(TIF_SIGPENDING);

this can't be right or I am totally confused.

Another signal_wake_up() can come right after clear(SIGPENDING).

Again, I'll try to re-read this patch, but let me ask anyway...

Do we have a plan B? I mean... iirc you have mentioned that you can
change these code paths to do something like

	if (killed)
		tell_the_drivers_that_all_callbacks_will_fail();

so that vhost_worker() can exit after get_signal() returns SIGKILL.

Probably I misunderstood you, but it would be nice to avoid the changes
in coredump/etc code just to add a temporary (iiuc!) fix.

Oleg.