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

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Message-ID: <20180722190035.ehav25wwwtz3fxfm@pengutronix.de>
Date:   Sun, 22 Jul 2018 21:00:35 +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

Hello,

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?

> Note also that g2 IRQ freeing gets the ordering right, and doesn't need
> a lock because it doesn't program any registers when tearing down it's
> IRQ.

Yes.

> Applied and queued up for -stable, thanks.

Fine, thanks
Uwe

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