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Subject: Re: [PATCH] tty: serial: owl: Fix data race in owl_uart_remove
To:     Johan Hovold <johan@kernel.org>
Cc:     Greg KH <gregkh@linuxfoundation.org>, jirislaby@kernel.org,
        linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
        ldv-project@linuxtesting.org, andrianov@ispras.ru
References: <20210617110443.6526-1-saubhik.mukherjee@gmail.com>
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 <YNLfxMZZ0a80qKLg@hovoldconsulting.com>
 <a9d43126-acd7-efb0-bf1a-86b06965f0e2@gmail.com>
 <YNR6uEbCJOa9s3hG@hovoldconsulting.com>
From:   Saubhik Mukherjee <saubhik.mukherjee@gmail.com>
Message-ID: <ff208383-aefa-17d0-891c-ee5a10edd1fe@gmail.com>
Date:   Fri, 25 Jun 2021 13:22:11 +0530
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On 6/24/21 5:59 PM, Johan Hovold wrote:
> On Thu, Jun 24, 2021 at 05:37:38PM +0530, Saubhik Mukherjee wrote:
>> On 6/23/21 12:46 PM, Johan Hovold wrote:
>>> On Wed, Jun 23, 2021 at 11:06:53AM +0530, Saubhik Mukherjee wrote:
>>>> On 6/17/21 4:52 PM, Greg KH wrote:
>>>>> On Thu, Jun 17, 2021 at 04:34:43PM +0530, Saubhik Mukherjee wrote:
>>>>>> Suppose the driver is registered and a UART port is added. Once an
>>>>>> application opens the port, owl_uart_startup is called which registers
>>>>>> the interrupt handler owl_uart_irq.
>>>>>>
>>>>>> We could have the following race condition:
>>>>>>
>>>>>> When device is removed, owl_uart_remove is called, which calls
>>>>>> uart_remove_one_port, which calls owl_uart_release_port, which writes
>>>>>> NULL to port->membase. At this point parallely, an interrupt could be
>>>>>> handled by owl_uart_irq which reads port->membase.
>>>>>>
>>>>>> This is because it is possible to remove device without closing a port.
>>>>>> Thus, we need to check it and call owl_uart_shutdown in owl_uart_remove.
>>>
>>> No, this makes no sense at all. The port is deregistered and hung up by
>>> uart_remove_one_port() (and the interrupt line is consequently disabled
>>> by the driver) before it is released so this can never happen.
>>
>> Thanks for the reply. I am not sure I understand. I could not find any
>> interrupt disabling in owl_uart_remove. Could you point out where/how is
>> the interrupt line is disabled before releasing the port?
> 
> The interrupt line is disabled by owl_uart_shutdown(), which is called
> when uart_remove_one_port() hangs up an open tty. And as I mentioned
> this happens after deregistering the port (so no new opens) and before
> releasing the port.
> 
> Johan
> 

Thank you very much for the explanation. So, indeed shutdown is called 
before releasing port. There is no need for a patch.