linux-kernel - Re: [PATCH] vt: Rework the console spawning variables.

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Date:	Sun, 10 Sep 2006 23:01:58 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	Linux Containers <containers@...ts.osdl.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] vt: Rework the console spawning variables.

Oleg Nesterov <oleg@...sign.ru> writes:

> On 09/10, Eric W. Biederman wrote:
>>
>> Oleg Nesterov <oleg@...sign.ru> writes:
>> 
>> > On 09/10, Eric W. Biederman wrote:
>> >>
>> >> Updating this old code is painful.
>> >
>> > No, no, we shouldn't change the old code, it is fine.
>> >
>> So what happens when:
>> cpu0:                         cpu1:
>> kill_pid(vt_pid,....)  fn_SAK()->vc_reset()->put_pid(xchg(&vt_pid, NULL))
>> 
>> Can't kill_pid dereference vt_pid after put_pid is called?
>
> Ah, I didn't consider that patch as 'old code', sorry :)

What I meant was that updating code that predates SMP support is painful.
When you said everything was ok. I was confused.

> I don't understand drivers/char/vt*, but surely put_pid(xchg()) can't work.
> Again, unless we have a lock to serialize access to ->vt_pid, but in that
> case we don't need xchg().

Ok.  So we are in violent agreement then, my patch was wrong.

The xchg half works.  For taking and putting a reference it is fine, you
just can't use that reference for anything safely.

So I need to come up with a new patch that gets it's locking correct,
in the fn_SAK case.

Eric
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