netdev - Re: [PATCH RFC] spinlock: split out debugging check from spin_lock

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Message-ID: <20130413120339.GA8168@neilslaptop.think-freely.org>
Date:	Sat, 13 Apr 2013 08:03:39 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Bart Van Assche <bvanassche@....org>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH RFC] spinlock: split out debugging check from
 spin_lock_mutex

On Sat, Apr 13, 2013 at 09:35:07AM +0200, Bart Van Assche wrote:
> On 04/12/13 20:45, Neil Horman wrote:
> >On Fri, Apr 12, 2013 at 04:01:04PM +0200, Bart Van Assche wrote:
> >>On 04/12/13 13:32, Neil Horman wrote:
> >>I think there is another issue with invoking mutex_trylock() and mutex_unlock()
> >>from IRQ context: as far as I can see if CONFIG_DEBUG_MUTEXES is disabled
> >>__mutex_unlock_common_slowpath() uses spin_lock() to lock mutex.wait_lock and
> >>hence invoking mutex_unlock() from both non-IRQ and IRQ context is not safe.
> >>Any thoughts about that ?
> >>
> >Yeah, its ugly, but in this specific case, its ok.  the netpoll code (in
> >netpoll_send_skb disables irq on the local cpu before entering the netpoll code
> >path any further, so whenver we frob this mutex from the local cpu, we're
> >guaranteed not to get pre-empted by an irq.
> 
> As far as I know it is neither allowed nor safe to call
> netpoll_rx_disable() with IRQs disabled. But that function can lock
Where do you see netpoll_rx_disable getting called with irqs disabled?

> the dev_lock mutex. What do you think will happen with
> CONFIG_DEBUG_MUTEXES=n if an interrupt occurs during the
> mutex_lock(&ni->dev_lock) call, that mutex_lock() call has already
> locked the mutex-internal spin lock via spin_lock() and
> mutex_trylock() is invoked from inside the interrupt ? Can that
> result in anything else than deadlock and "CPU stuck" messages ?
> 
Please go back and look more closely.  Where do you see a mutex_lock call
getting made with interrupts enabled?

Neil

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