Re: [PATCH v2 4/4] copy_signal cleanup: clean tty_audit_fork()

[Oleg Nesterov <oleg@redhat.com>]

On 12/05, Miloslav Trmac wrote:
>
> ----- "Oleg Nesterov" <oleg@redhat.com> wrote:
> > On 12/04, Veaceslav Falico wrote:
> > > diff --git a/drivers/char/tty_audit.c b/drivers/char/tty_audit.c
> > > index ac16fbe..283a15b 100644
> > > --- a/drivers/char/tty_audit.c
> > > +++ b/drivers/char/tty_audit.c
> > > @@ -148,7 +148,6 @@ void tty_audit_fork(struct signal_struct *sig)
> > >  	spin_lock_irq(&current->sighand->siglock);
> > >  	sig->audit_tty = current->signal->audit_tty;
> > >  	spin_unlock_irq(&current->sighand->siglock);
> > > -	sig->tty_audit_buf = NULL;
> > >  }
> >
> > Off-topic question to this who understands this code.
> >
> > But afaics we can also remove ->siglock from this helper and make
> > it really trivial for being inline. ->siglock buys nothing, we just
> > read a boolean. In fact, after the quick grep I do not understand
> > how ->siglock is connected to ->audit_tty. OK, it protects
> > tty_audit_buf,
> > but why we always take ->siglock to access ->audit_tty ?
> AFAIK there is no explicit documentation of the atomicity semantics
> expected by the Linux kernel (both from the hardware and from the compiler),
> so every access to the boolean is protected by a lock, to be on the safe side.

Not sure I understand, but the kernel relies on fact it is always safe
to load/store a word.

What atomicity semantics do you mean and how ->siglock can help? Sure,
we can race with AUDIT_TTY_SET, but this can happen with or without
this lock. This "race" is unavoidable and harmless.

I believe every spin_lock(siglock) around ->audit_tty is bogus.

Oleg.