Re: crash in tty code in 6.1.. fixed since?

Hans Petter Selasky wrote:
> On Friday 13 July 2007 10:25, Attilio Rao wrote:
>> Hans Petter Selasky wrote:
>>> If TTY was not Giant locked, you would have had an error there if another
>>> mutex was locked, and the problem would have been solved years ago :-)
>> Not sure what you mean, but the first evidence is that you would have
>> explicitly drop/pickup the mutex so that you would have handled the race
>>   not trasparently as Giant does.
>> Moreover, it seems that tty should be partially locked with a sleeping
>> primitive (sx probabilly).
>>
> 
> If you lock a mutex first and then a sx-lock, you should get a warning, right?

No, a panic.

What I mean is that if you had a mutex here instead than Giant what 
would have probabilly happened is having code like this:

mtx_lock(&tty_mtx);
...
if (tp->t_session) {
	mtx_unlock(&tty_mtx);
	sx_slock(&proctree_lock);
	mtx_lock(&tty_mtx);
	if (tp->t_session && tp->t_session->s_leader) {
		struct proc *p;

What changes really here is that you explicitly check again the state of 
t_session ptr since it can be changed while dropping/pickingup again the 
tty_mtx. Since you used a mutex differently from Giant you know you have 
to do that.
With Giant the problem is that the dropping/pickingup happens implicitly 
in our primitives so you can just make (easy) mistakes like these.

Attilio