On Thu, 26 Jun 2014, Nicholas A. Bellinger wrote:
> Hi Mikulas,
>
> On Mon, 2014-06-23 at 13:42 -0400, Mikulas Patocka wrote:
> > target: fix deadlock on unload
> >
> > On uniprocessor preemptible kernel, target core deadlocks on unload. The
> > following events happen:
> > * iscsit_del_np is called
> > * it calls send_sig(SIGINT, np->np_thread, 1);
> > * the scheduler switches to the np_thread
> > * the np_thread is woken up, it sees that kthread_should_stop() returns
> > false, so it doesn't terminate
> > * the np_thread clears signals with flush_signals(current); and goes back
> > to sleep in iscsit_accept_np
> > * the scheduler switches back to iscsit_del_np
> > * iscsit_del_np calls kthread_stop(np->np_thread);
> > * the np_thread is waiting in iscsit_accept_np and it doesn't respond to
> > kthread_stop
> >
> > The deadlock could be resolved if the administrator sends SIGINT signal to
> > the np_thread with killall -INT iscsi_np
> >
> > The reproducible deadlock was introduced in commit
> > db6077fd0b7dd41dc6ff18329cec979379071f87, but the thread-stopping code was
> > racy even before.
> >
> > This patch fixes the problem. Using kthread_should_stop to stop the
> > np_thread is unreliable, so we test np_thread_state instead. If
> > np_thread_state equals ISCSI_NP_THREAD_SHUTDOWN, the thread exits.
> >
> > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> >
>
> Apologies for the delayed response..
>
> Applied to target-pending/master and including in the next -rc3 PULL
> request.
>
> Also FYI, I've added '3.12+' to the stable tag to match how far back
> commit db6077fd0 has been included in stable.
>
> Thanks,
>
> --nab
Hi
I think db6077fd0 should be backported to stable kernels beginning with
3.10 (because they set np->np_thread = NULL in
__iscsi_target_login_thread). The current 3.10-stable branch misses this
patch.
This patch for unload deadlock should be backported to all stable kernels
(because unload is racy there), but because of different code, we should
make a different patch for old stable branches.
For example in 3.4.95, __iscsi_target_login_thread contains this code:
spin_lock_bh(&np->np_thread_lock);
if (np->np_thread_state == ISCSI_NP_THREAD_RESET) {
np->np_thread_state = ISCSI_NP_THREAD_ACTIVE;
complete(&np->np_restart_comp);
} else {
np->np_thread_state = ISCSI_NP_THREAD_ACTIVE;
}
spin_unlock_bh(&np->np_thread_lock);
If the state is ISCSI_NP_THREAD_SHUTDOWN, the above piece of code will
change it to ISCSI_NP_THREAD_ACTIVE and open the same kthread_should_stop
race.
Mikulas
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