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

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Message-Id: <20180722.130411.1782941097108136413.davem@davemloft.net>
Date:   Sun, 22 Jul 2018 13:04:11 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     u.kleine-koenig@...gutronix.de
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

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.