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Subjectping: drivers/staging/usbip/ abuses task_is_dead/exit_state
On 09/20, Greg KH wrote:
>
> On Tue, Sep 20, 2011 at 05:14:10PM +0200, Oleg Nesterov wrote:
> > (add more cc's)
> >
> > On 09/20, Oleg Nesterov wrote:
> > >
> > > Unfortunately, we can't kill task_is_dead() right now, it has already
> > > found the users in drivers/staging/, and I bet the usage is wrong.
> >
> > It is used by drivers/staging/usbip/
> >
> > For what? The code:
> >
> > if (vdev->ud.tcp_rx && !task_is_dead(vdev->ud.tcp_rx))
> > kthread_stop(vdev->ud.tcp_rx);
> >
> > And how task_is_dead() can help? This helper is really "special", it
> > shouldn't be used anyway. But why do we check ->exit_state? Without
> > tasklist the check is racy anyway, the task can exit right after the
> > check.
> >
> > And. It is safe to use kthread_stop(t) even if t has already exited.
> >
> > OK, this was added by 8547d4cc2b616e4f1dafebe2c673fc986422b506
> > "Staging: usbip: vhci-hcd: Do not kill already dead RX/TX kthread"
> >
> > When unbinding a device on the host which was still attached on the
> > client, I got a NULL pointer dereference on the client.
> >
> > Where?
> >
> > This turned out
> > to be due to kthread_stop() being called on an already dead kthread.
> >
> > This should work.
> >
> > I'm afraid this can only fix the symptom. Probably, the problem is that
> > we do not have the reference and thus even task_is_dead(t) is not safe.
> >
> > This kthread was created by kthread_run(). If it exits, nothing protects
> > this task_struct.
> >
> > In any case, please do not use ->exit_state. It should not be used outside
> > of exit.c/etc paths, "exit_state != 0" means "exit_notify() was called".
>
> Patches to fix this up in this driver are always gladly appreciated :)

OK, since nobody cares, probably I should make the patch even if I don't
understand this code at all and can't test the change.

But, Tobias, may be you can explain what this task_is_dead() check was
supposed to do?

Oleg.



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