linux-kernel - Re: [PATCH v6] usb: gadget: u_serial: Add null pointer check in gs_read_complete & gs_write

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Message-ID: <8ab0ca38-1bf4-ed3e-eef0-cbed2a524b34@quicinc.com>
Date: Fri, 23 Aug 2024 13:04:18 +0530
From: Prashanth K <quic_prashk@...cinc.com>
To: Michael Nazzareno Trimarchi <michael@...rulasolutions.com>,
        胡连勤 <hulianqin@...o.com>
CC: "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "quic_jjohnson@...cinc.com" <quic_jjohnson@...cinc.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        opensource.kernel <opensource.kernel@...o.com>,
        "akpm@...ux-foundation.org"
	<akpm@...ux-foundation.org>
Subject: Re: [PATCH v6] usb: gadget: u_serial: Add null pointer check in
 gs_read_complete & gs_write_complete



On 23-08-24 12:28 pm, Michael Nazzareno Trimarchi wrote:
> Hi
> 
> On Fri, Aug 23, 2024 at 8:40 AM 胡连勤 <hulianqin@...o.com> wrote:
>>
>> Hello linux community expert:
>>
>>>> Fixes: c1dca562be8a ("usb gadget: split out serial core")
>>>> Cc: stable@...r.kernel.org
>>>> Signed-off-by: Lianqin Hu <hulianqin@...o.com>
>>>> ---
>>>> v6:
>>>>   - Update the commit text
>>>>   - Add the Fixes tag
>>>>   - CC stable kernel
>>>>   - Add serial_port_lock protection when checking port pointer
>>>>   - Optimize code comments
>>>>   - Delete log printing
>>
>>> You need to list ALL of the versions here, I seem to have missed v4 and
>>> v5 somewhere so I don't know what changed there.
>>

[...]
>>> nested spinlocks, why?  Did you run this with lockdep enabled to verify you aren't hitting a different bug now?
>>  Because there is a competition relationship between this function and the gserial_disconnect function,
>>  the gserial_disconnect function first obtains serial_port_lock and then obtains port->port_lock.
>>  The purpose of nesting is to ensure that when gs_read_complete is executed, it can be successfully executed after obtaining serial_port_lock.
>>  gserial_disconnect(..)
>>  {
>>         struct gs_port  *port = gser->ioport;
>>         ...
>>         spin_lock_irqsave(&serial_port_lock, flags);
>>         spin_lock(&port->port_lock);
>>         ...
>>         gser->ioport = NULL;   ---> port = NULL;
>>         ...
>>         spin_unlock(&port->port_lock);
>>         spin_unlock_irqrestore(&serial_port_lock, flags);
>>  }
>>
>> After enabling the lockdep function (CONFIG_DEBUG_LOCK_ALLOC=y), there is no lockdep-related warning information.
>>
>>> And why is one irqsave and one not?  That feels odd, it might be right, but you need to document here why the difference.
>>  After the gs_read_complete function is executed, spin_unlock_irqrestore is used to restore the previous state，
> 
> 胡连勤 this is not a common locking pattern that is the reason that
> should be properly described.
This pattern was already used on gser_suspend/resume callbacks, this was
done because the lock was stored under port (and port itself was
becoming null), hence we added a static spinlock to mitigate it.
> 
>> -       /* Queue all received data until the tty layer is ready for it. */
>>         spin_lock(&port->port_lock);
>> +       spin_unlock(&serial_port_lock);
>> +
>> +       /* Queue all received data until the tty layer is ready for it. */
>>         list_add_tail(&req->list, &port->read_queue);
>>         schedule_delayed_work(&port->push, 0);
>> -       spin_unlock(&port->port_lock);
>> +       spin_unlock_irqrestore(&port->port_lock, flags);   ---> Here we use spin_unlock_irqrestore to restore the state
>>  }
>>
>> Thanks
> 
> Thank you