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

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Date:   Mon, 22 May 2023 12:00:39 -0500
From:   Mike Christie <michael.christie@...cle.com>
To:     Oleg Nesterov <oleg@...hat.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 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps
 regression

On 5/22/23 7:30 AM, Oleg Nesterov wrote:
> Confused, please help...
> 
> On 05/21, Mike Christie wrote:
>>
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -338,6 +338,7 @@ static int vhost_worker(void *data)
>>  	struct vhost_worker *worker = data;
>>  	struct vhost_work *work, *work_next;
>>  	struct llist_node *node;
>> +	bool dead = false;
>>
>>  	for (;;) {
>>  		/* mb paired w/ kthread_stop */
>> @@ -349,8 +350,22 @@ static int vhost_worker(void *data)
>>  		}
>>
>>  		node = llist_del_all(&worker->work_list);
>> -		if (!node)
>> +		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.
>> +			 */
> 
> But who will call the release function / vhost_task_stop() and when this
> will happen after this thread gets SIGKILL ?

When we get a SIGKILL, the thread that owns the device/vhost_task will
also exit since it's the same thread group and it does:

do_exit -> exit_files -> put_files_struct -> close_files -> fput

which eventually (the actual put is done via the delayed work path
in there) runs the file_operation->release.

So the release function is being called in parallel more or less as the
code above.

> 
>> +			if (!dead && signal_pending(current)) {
>> +				struct ksignal ksig;
>> +
>> +				dead = get_signal(&ksig);
>> +				if (dead)
>> +					clear_thread_flag(TIF_SIGPENDING);
> 
> If you do clear_thread_flag(TIF_SIGPENDING), then why do we need 1/3 ?

You're right. I don't need it. I thought __fatal_signal_pending checked
the shared pending as well but it only checks pending.

> Also. Suppose that vhost_worker() dequeues SIGKILL and clears TIF_SIGPENDING.
> 
> SIGSTOP, PTRACE_INTERRUPT, freezer can come and set TIF_SIGPENDING again.
> In this case the main for (;;) loop will spin without sleeping until
> vhost_task_should_stop() becomes true?

I see. So I either have to be able to call get_signal after SIGKILL or
at this time work like a kthread and ignore signals like a

if (dead && signal_pending())
	flush_signals()
?