linux-kernel - Re: RCU stall in 8250 serial driver Linux 4.15-rc1

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Date:   Tue, 23 Jan 2018 15:52:29 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Shankara Pailoor <sp3485@...umbia.edu>
Cc:     Greg KH <gregkh@...uxfoundation.org>, jslaby@...e.com,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>,
        linux-serial@...r.kernel.org
Subject: Re: RCU stall in 8250 serial driver Linux 4.15-rc1

On Wed, 17 Jan 2018 09:24:32 -0800
Shankara Pailoor <sp3485@...umbia.edu> wrote:

> Hi Greg,
> 
> Sorry for that. Here is the stack trace. C Program below

>  serial_in drivers/tty/serial/8250/8250.h:111 [inline]
>  wait_for_xmitr+0x8a/0x1d0 drivers/tty/serial/8250/8250_port.c:2033
>  serial8250_console_putchar+0x19/0x50 drivers/tty/serial/8250/8250_port.c:3170
>  uart_console_write+0x98/0xc0 drivers/tty/serial/serial_core.c:1858

The console is spinning in polled mode trying to write data to the
console port, which it seems has gotten a bit stuck. Could be the
hypervisor hits some kind of buffering limit, could be hypervisor
interface broke.

Either way the console interface is supposed to stall the machine to
ensure the bytes always get out and if your serial port jams or gets
massively behind then this will happen.

Alan