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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/