netdev - Re: [PATCH] net: dsa: mv88e6xxx: fix races between lock and irq freeing

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Date:   Sun, 22 Jul 2018 22:38:26 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     David Miller <davem@...emloft.net>
Cc:     andrew@...n.ch, vivien.didelot@...oirfairelinux.com,
        f.fainelli@...il.com, netdev@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH] net: dsa: mv88e6xxx: fix races between lock and irq
 freeing

On Sun, Jul 22, 2018 at 01:04:11PM -0700, David Miller wrote:
> From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> Date: Sun, 22 Jul 2018 21:00:35 +0200
> 
> > On Sat, Jul 21, 2018 at 10:44:09PM -0700, David Miller wrote:
> >> From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> >> Date: Fri, 20 Jul 2018 11:53:15 +0200
> >> 
> >> > free_irq() waits until all handlers for this IRQ have completed. As the
> >> > relevant handler (mv88e6xxx_g1_irq_thread_fn()) takes the chip's reg_lock
> >> > it might never return if the thread calling free_irq() holds this lock.
> >> > 
> >> > For the same reason kthread_cancel_delayed_work_sync() in the polling case
> >> > must not hold this lock.
> >> > 
> >> > Also first free the irq (or stop the worker respectively) such that
> >> > mv88e6xxx_g1_irq_thread_work() isn't called any more before the irq
> >> > mappings are dropped in mv88e6xxx_g1_irq_free_common() to prevent the
> >> > worker thread to call handle_nested_irq(0) which results in a NULL-pointer
> >> > exception.
> >> > 
> >> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> >> 
> >> Looks good.
> >> 
> >> Note than the IRQ domain unmapping will do a synchronize_irq() which
> >> should cause the same deadlock as free_irq() will with the reg_lock
> >> held.
> > 
> > Do you think that there is still a problem? When free_irq() for the
> > external visible irq returns the muxed irqs should be all gone, too, so
> > this should not trigger, should it?
> 
> It shouldn't be a problem after your changes.
> 
> I'm just saying that I'm surprised that, in the original code, you see
> the deadlock in free_irq(), since the synchronize_irq() done by the
> IRQ domain code should have happened first.

ah, I see. This didn't happen because I added an msleep to
mv88e6xxx_g1_irq_thread_work() before the lock it taken to widen the
race window for a different problem. So the sub-irqs were not active
when mv88e6xxx_g1_irq_free() run, only the mux-irq was. When
irq_dispose_mapping() is called for the sub-irq there is no problem as
this results in synchronize_irq() for the sub-irq, not the mux-irq.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |