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Subject: Re: [PATCH v3 3/3] uio: fix crash after the device is unregistered
To:     Mike Christie <mchristi@redhat.com>, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org
Cc:     hamish.martin@alliedtelesis.co.nz, jannh@google.com,
        pkalever@redhat.com, pkarampu@redhat.com, atumball@redhat.com,
        sabose@redhat.com
References: <1530845836-49101-1-git-send-email-xiubli@redhat.com>
 <1530845836-49101-4-git-send-email-xiubli@redhat.com>
 <5B3FB3B0.7010105@redhat.com>
 <1d3a72f6-2d48-1e36-82e4-c764c5359765@redhat.com>
 <5B439618.5060800@redhat.com>
From:   Xiubo Li <xiubli@redhat.com>
Message-ID: <4f39f032-6ab4-2dff-346a-9c752aa16ed5@redhat.com>
Date:   Tue, 10 Jul 2018 10:40:17 +0800
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On 2018/7/10 1:06, Mike Christie wrote:
> On 07/06/2018 08:28 PM, Xiubo Li wrote:
>> On 2018/7/7 2:23, Mike Christie wrote:
>>> On 07/05/2018 09:57 PM, xiubli@redhat.com wrote:
>>>>    static irqreturn_t uio_interrupt(int irq, void *dev_id)
>>>>    {
>>>>        struct uio_device *idev = (struct uio_device *)dev_id;
>>>> -    irqreturn_t ret = idev->info->handler(irq, idev->info);
>>>> +    irqreturn_t ret;
>>>> +
>>>> +    mutex_lock(&idev->info_lock);
>>>> +    if (!idev->info) {
>>>> +        ret = IRQ_NONE;
>>>> +        goto out;
>>>> +    }
>>>>    +    ret = idev->info->handler(irq, idev->info);
>>>>        if (ret == IRQ_HANDLED)
>>>>            uio_event_notify(idev->info);
>>>>    +out:
>>>> +    mutex_unlock(&idev->info_lock);
>>>>        return ret;
>>>>    }
>>> Do you need the interrupt related changes in this patch and the first
>>> one?
>> Actually, the NULL checking is not a must, we can remove this. But the
>> lock/unlock is needed.
>>>    When we do uio_unregister_device -> free_irq does free_irq return
>>> when there are no longer running interrupt handlers that we requested?
>>>
>>> If that is not the case then I think we can hit a similar bug. We do:
>>>
>>> __uio_register_device -> device_register -> device's refcount goes to
>>> zero so we do -> uio_device_release -> kfree(idev)
>>>
>>> and if it is possible the interrupt handler could still run after
>>> free_irq then we would end up doing:
>>>
>>> uio_interrupt -> mutex_lock(&idev->info_lock) -> idev access freed
>>> memory.
>> I think this shouldn't happen. Because the free_irq function does not
>> return until any executing interrupts for this IRQ have completed.
>>
> If free_irq returns after executing interrupts and does not allow new
> executions what is the lock protecting in uio_interrupt?
>
I meant idev->info->handler(irq, idev->info), it may should be protected 
by the related lock in each driver.

Thanks，