linux-kernel - Re: [PATCH] target: fix deadlock on unload

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Date:	Fri, 27 Jun 2014 08:35:03 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
cc:	linux-scsi@...r.kernel.org, target-devel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] target: fix deadlock on unload



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@...hat.com>
> > Cc: stable@...r.kernel.org
> > 
> 
> 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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