Re: [PATCH] usb: gadget: u_serial: Add null pointer check in gserial_resume

[Prashanth K]

On 09-02-23 08:39 pm, Alan Stern wrote:
> On Thu, Feb 09, 2023 at 07:37:01PM +0530, Prashanth K wrote:
> > On 09-02-23 12:33 pm, Prashanth K wrote:
> > > On 09-02-23 12:31 pm, Greg Kroah-Hartman wrote:
> > > > On Thu, Feb 09, 2023 at 10:31:50AM +0530, Prashanth K wrote:
> > > > > In that case i guess we have to make port_lock a global variable
> > > > > and take it
> > > > > out of gs_port structure.
> > > > >
> > > > > + static DEFINE_SPINLOCK(port_lock);
> > > > >
> > > > > struct gs_port {
> > > > >      struct tty_port port;
> > > > > -    spinlock_t port_lock;
> > > > >
> > > > > This will require us to change all the spinlock(port->port_lock) used in
> > > > > u_serial.c, what do you suggest?
> > > >
> > > > Yes, that would be the correct thing to do.
> > Hi Greg/Alan, One general doubt, if we make the spinlock static/global,
> > wouldn't that be a problem when there are multiple instances, and also
> > multiple interfaces can use u_serial at same time. Asking this because
> > u_serial can be used by f_serial (gser) as well as f_acm (acm).
>
> You should consider having _two_ spinlocks: One in the gs_port structure
> (the way it is now) and a separate global lock.  The first would be used
> in situations where you know you have a valid pointer.  The second would
> be used in situations where you don't know if the pointer is non-NULL
> or where you are changing the pointer's value.
Lets say we replaced the existing spinlock in gserial_resume and 
gserial_disconnect with a new static spinlock, and kept the spinlocks in 
other functions unchanged. In that case, wouldn't it cause additional 
race conditions as we are using 2 different locks.

Thanks,
Prashanth K
> Alan Stern