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Date:   Mon, 5 Jun 2023 15:26:11 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Mike Christie <michael.christie@oracle.com>, linux@leemhuis.info,
        nicolas.dichtel@6wind.com, axboe@kernel.dk,
        torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, mst@redhat.com,
        sgarzare@redhat.com, jasowang@redhat.com, stefanha@redhat.com,
        brauner@kernel.org
Subject: Re: [PATCH 1/1] fork, vhost: Use CLONE_THREAD to fix freezer/ps
 regression
Message-ID: <20230605132611.GB32275@redhat.com>
References: <20230601183232.8384-1-michael.christie@oracle.com>
 <20230602192254.GD555@redhat.com>
 <87wn0l2or4.fsf@email.froward.int.ebiederm.org>
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On 06/02, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
>
> > 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).
>
> Technically yes.

...

> Beyond that clearing TIF_SIGPENDING is just an optimization so
> the thread can sleep in schedule and not spin.

Yes. So if another signal_wake_up() comes after clear(SIGPENDING)
this code will spin in busy-wait loop waiting VHOST_TASK_FLAGS_STOP.
Obviously not good and even deadlockable on UP && !PREEMPT.

Oleg.