Re: [PATCH] tty/hvc_console: fix console lock ordering with spinlock

[Michael Ellerman]

Denis Kirjanov <k...@linux-powerpc.org> writes:

> hvc_remove() takes a spin lock first then acquires the console
> semaphore. This situation can easily lead to a deadlock scenario
> where we call scheduler with spin lock held.

Have you actually hit the deadlock? Because that code's been like that
for years and I've never received a bug report.

> diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
> index b19ae36..a8d3991 100644
> --- a/drivers/tty/hvc/hvc_console.c
> +++ b/drivers/tty/hvc/hvc_console.c
> @@ -920,17 +920,17 @@ int hvc_remove(struct hvc_struct *hp)
>  
>       tty = tty_port_tty_get(&hp->port);
>  
> +     console_lock();
>       spin_lock_irqsave(&hp->lock, flags);
>       if (hp->index < MAX_NR_HVC_CONSOLES) {
> -             console_lock();
>               vtermnos[hp->index] = -1;
>               cons_ops[hp->index] = NULL;
> -             console_unlock();
>       }
>  
>       /* Don't whack hp->irq because tty_hangup() will need to free the irq. 
> */
>  
>       spin_unlock_irqrestore(&hp->lock, flags);
> +     console_unlock();

I get that you're trying to do the minimal change, but I don't think the
result is ideal. If this isn't a console hvc then we take both locks but
do nothing.

So what about:

        if (hp->index < MAX_NR_HVC_CONSOLES) {
                console_lock();
                spin_lock_irqsave(&hp->lock, flags);
                vtermnos[hp->index] = -1;
                cons_ops[hp->index] = NULL;
                spin_unlock_irqrestore(&hp->lock, flags);
                console_unlock();
        }

cheers