Hi Prashanth
On Fri, Aug 23, 2024 at 9:34 AM Prashanth K <quic_prashk@xxxxxxxxxxx> 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@xxxxxxxx> wrote:
> >>
> >> Hello linux community expert:
> >>
> >>>> Fixes: c1dca562be8a ("usb gadget: split out serial core")
> >>>> Cc: stable@xxxxxxxxxxxxxxx
> >>>> Signed-off-by: Lianqin Hu <hulianqin@xxxxxxxx>
> >>>> ---
> >>>> 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@xxxxxxxxxxxxxxxxxxxx
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