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: <CAOf5uwmQ_drN9xE88Eyf01kmh=GLwitLCSo7KxiNDE9-h-D0KQ@mail.gmail.com>
Date: Fri, 23 Aug 2024 09:57:44 +0200
From: Michael Nazzareno Trimarchi <michael@...rulasolutions.com>
To: Prashanth K <quic_prashk@...cinc.com>
Cc: 胡连勤 <hulianqin@...o.com>, 
	"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

Hi Prashanth

On Fri, Aug 23, 2024 at 9:34 AM Prashanth K <quic_prashk@...cinc.com> wrote:
>
>
>
> 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.
> >
I see now, but why don't disable the endpoint before disconnecting?

/* disable endpoints, aborting down any active I/O */
usb_ep_disable(gser->out);
usb_ep_disable(gser->in);

Michael


> >> -       /* 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



-- 
Michael Nazzareno Trimarchi
Co-Founder & Chief Executive Officer
M. +39 347 913 2170
michael@...rulasolutions.com
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