Re: [RFC][PATCH] ->signal->tty locking

[Prarit Bhargava <prarit@redhat.com>]

Peter Zijlstra wrote:
> Oleg wrote:
> "Historically ->signal/->sighand (both ptrs and their contents) were globally
> protected by tasklist_lock. 'current' can use these pointers lockless, they
> can't be changed under him.
>
> Nowadays ->signal/->sighand are _also_ protected by ->sighand->siglock.
> Unless you are current, you can't lock ->siglock directly (without holding
> tasklist_lock), you should use lock_task_sighand()."
>
> Then, to be consistent with the rest of the kernel, ->signal->tty
> locking should look like so:
>
>   mutex_lock(&tty_mutex)
>     read_lock(&tasklist_lock)
>       lock_task_sighand(p, &flags)
>   

It would be nice if we could clean up some of the complicated locking in 
this code.  For example, from do_tty_hangup,

 *
 *      Locking:
 *              BKL
 *              redirect lock for undoing redirection
 *              file list lock for manipulating list of ttys
 *              tty_ldisc_lock from called functions
 *              termios_sem resetting termios data
 *              tasklist_lock to walk task list for hangup event

P.