linux-kernel - Re: UART/TTY console deadlock

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Message-ID: <20200630130534.GB145027@jagdpanzerIV.localdomain>
Date:   Tue, 30 Jun 2020 22:05:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Raul Rangel <rrangel@...gle.com>,
        Tony Lindgren <tony@...mide.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        kurt@...utronix.de, "S, Shirish" <Shirish.S@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        John Ogness <john.ogness@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: UART/TTY console deadlock

On (20/06/30 14:22), Petr Mladek wrote:
> On Tue 2020-06-30 19:55:12, Sergey Senozhatsky wrote:
> > On (20/06/30 12:21), Petr Mladek wrote:
> > > Anyway, I believe that this is a false positive. If I get it correctly
> > > serial8250_do_startup() must be called before the serial port could
> > > be registered as a console. It means that it could not be called
> > > from inside printk().
> > 
> > >From my understanding, I'm afraid we are talking about actual deadlock
> > here, not about false positive report. Quoting the original email:
> 
> IMHO, it is not clear.
>

Yea. Maybe it is, maybe it's not.

Regardless, after lockdep tells us there is something it doesn't like,
it disables itself, so false positives render lockdep useless.

> > > > @@ -2275,6 +2275,11 @@ int serial8250_do_startup(struct uart_port *port)
> > > >  
> > > >  	if (port->irq && !(up->port.flags & UPF_NO_THRE_TEST)) {
> > > >  		unsigned char iir1;
> > > > +		bool irq_shared = up->port.irqflags & IRQF_SHARED;
> > > > +
> > > > +		if (irq_shared)
> > > > +			disable_irq_nosync(port->irq);
> > > > +
> > > >  		/*
> > > >  		 * Test for UARTs that do not reassert THRE when the
> > > >  		 * transmitter is idle and the interrupt has already
> > > > @@ -2284,8 +2289,6 @@ int serial8250_do_startup(struct uart_port *port)
> > > >  		 * allow register changes to become visible.
> > > >  		 */
> > > >  		spin_lock_irqsave(&port->lock, flags);
> > > > -		if (up->port.irqflags & IRQF_SHARED)
> > > > -			disable_irq_nosync(port->irq);
> > > >  
> > > >  		wait_for_xmitr(up, UART_LSR_THRE);
> > > >  		serial_port_out_sync(port, UART_IER, UART_IER_THRI);
> > > > @@ -2297,9 +2300,9 @@ int serial8250_do_startup(struct uart_port *port)
> > > >  		iir = serial_port_in(port, UART_IIR);
> > > >  		serial_port_out(port, UART_IER, 0);
> > > >  
> > > > -		if (port->irqflags & IRQF_SHARED)
> > > > -			enable_irq(port->irq);
> > > >  		spin_unlock_irqrestore(&port->lock, flags);
> > > > +		if (irq_shared)
> > > > +			enable_irq(port->irq);
> > > >  
> > > >  		/*
> > > >  		 * If the interrupt is not reasserted, or we otherwise
> > > 
> > > I think that it might be safe but I am not 100% sure, sigh.
> > 
> > Yeah, I'm not 100%, but I'd give it a try.
> 
> I do not feel brave enough to ack it today. But I am all for trying it
> if anyone more familiar with the code is fine with it.

I see. Well, I suppose we need Ack-s from tty/serial/8250 maintainers.
I would not be very happy if _only_ printk people Ack the patch.

	-ss