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

[ebiederm@xmission.com (Eric W. Biederman)]

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

> On 09/10, Eric W. Biederman wrote:
>>
>> Ok.  I think I see the where the confusion is.  We were looking
>> at different parts of the puzzle.  But I we need to resolve this
>> to make certain I didn't do something clever and racy.
>
> Yes, I think we misunderstood each other :)
>
>> As for the rest of your suggestion it would not be hard to be able to
>> follow a struct pid pointer in an rcu safe way, and we do in the pid
>> hash table.  In other contexts so far I always have other variables
>> that need to be updated in concert, so there isn't a point in coming
>> up with a lockless implementation.  I believe vt_pid is the only
>> case that I have run across where this is a problem and I have
>> at least preliminary patches for every place where signals are
>> sent.
>> 
>> 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?

It's a microscopic window, and requires the user to attempt a vt switch
and a sak simultaneously but I think it is there.

Eric