linux-kernel - Re: [PATCH] vt: Make SAK run in process context.

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Message-ID: <m1irgit96b.fsf@ebiederm.dsl.xmission.com>
Date:	Mon, 11 Dec 2006 14:27:40 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...l.org>
Cc:	Linux Containers <containers@...ts.osdl.org>,
	<linux-kernel@...r.kernel.org>, Oleg Nesterov <oleg@...sign.ru>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] vt: Make SAK run in process context.

Andrew Morton <akpm@...l.org> writes:

> On Mon, 11 Dec 2006 06:07:03 -0700
> ebiederm@...ssion.com (Eric W. Biederman) wrote:
>
>> 
>> This defers SAK so we can use the normal console semaphore to order
>> operations.
>> 
>> This removes the xchg operations that I used to attempt to attmically
>> update struct pid, because of the strange locking used for SAK.  With
>> SAK using the normal console semaphore nothing special is needed.
>> 
>
> This is all a bit smelly.

Ok.  I will take a second look, thanks for catching this.

I think I was half blind when I prepared this patch, I missed
that do_SAK was scheduling work itself.

>>  
>> +void deferred_SAK(void *data)
>> +{
>> +	struct vc *vc_con = data;
>> +	struct vc_data *vc;
>> +	struct tty_struct *tty;
>> +	
>> +	acquire_console_sem();
>> +	vc = vc_con->d;
>> +	if (vc) {
>> +		tty = vc->vc_tty;
>> +		/*
>> +		 * SAK should also work in all raw modes and reset
>> +		 * them properly.
>> +		 */
>> +		if (tty)
>> +			do_SAK(tty);
>> +		reset_vc(vc);
>> +	}
>> +	release_console_sem();
>> +}
>
> And a workqueue callback which calls a function which immediately does
> another schedule_work().
>
> I suspect you can fix all of this by passing a function pointer into
> do_SAK(): to either __do_SAK or to some new function which does the vc
> lookup then calls __do_SAK().

Yes.  It looks like all I need is an appropriate factor of __do_SAK() that
I can call immediately.  

> It probably means that you'll need to pass some payload into the workqueue
> callback, and dhowells just went and broke that on us.  That can be fixed
> by adding a new `void *tty_struct.SAK_work_data'.  
>
>
> hmm, do_SAK() is being a bit bad, overwriting the ->SAK_work on a
> work_struct which might presently be scheduled.  To do this safely we need
> a new variant on queue_work():

And of course there is the truly silly issue that X spells uses
Ctrl-Alt-Backspace instead of the kernel provided SAK to implement this.

Regardless that looks right.  Unless there is some locking on the tty we
can exploit.

> int queue_work_with_data(struct workqueue_struct *wq,
> 			struct work_struct *work, void **datap, void *data
> {
> 	int ret = 0, cpu = get_cpu();
>
> 	if (!test_and_set_bit(WORK_STRUCT_PENDING, &work->management)) {
> 		if (datap)
> 			*datap = data;
> 		if (unlikely(is_single_threaded(wq)))
> 			cpu = singlethread_cpu;
> 		BUG_ON(!list_empty(&work->entry));
> 		__queue_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
> 		ret = 1;
> 	}
> 	put_cpu();
> 	return ret;
> }
>
> then, of course,
>
> int queue_work(struct workqueue_struct *wq, struct work_struct *work)
> {
> 	return queue_work_with_data(wq, work, NULL, NULL);
> }
>
> iirc, other places in the kernel need queue_work_with_data(), for removal
> of the *_WORK_NAR() stuff.

Wow.  The intersection of the clean ups.

Eric

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