On 12/03, Andrew Morton wrote:
>
> On Sat, 2 Dec 2006 02:48:26 +0300
> Oleg Nesterov <[email protected]> wrote:
>
> > drivers/char/vt_ioctl.c changes vc->vt_pid doing
> >
> > put_pid(xchg(&vc->vt_pid, ...));
> >
> > This is unsafe, put_pid() can actually free the memory while vc->vt_pid is
> > still used by kill_pid(vc->vt_pid).
> >
> > Add a new helper, put_pid_rcu(), which frees "struct pid" via rcu callback
> > and convert vt_ioctl.c to use it.
> >
>
>
> I'm a bit reluctant to go adding more tricky infrastructure (especially
> 100% undocumented infrastructure) on behalf of a single usage site in a
> place as creepy as the VT ioctl code.
> If we envisage future users of this infrastructure (and if it gets
> documented) then OK.
It is a shame we can't use "struct pid*" lockless, note that "struct pid"
itself is rcu-protected. I hope we can find another usage for put_pid_rcu
(in fact I suggested it before, but didn't have a reason). However, I don't
see any other example immediately.
> Otherwise I'd rather just stick another bandaid into
> the vt code. Can we add some locking there,
Yes, this is possible, and probably we should do just this.
> or change it to use a
> task_struct* or something?
I don't think this is good. It was converted from task_struct* to pid*.
Eric, what do you think?
Oleg.
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