Date:	Sat, 13 Nov 2010 07:18:30 +0300
From:	Alexey Zaytsev <alexey.zaytsev@...il.com>
To:	Greg Kroah-Hartman <gregkh@...e.de>
Cc:	linux-kernel@...r.kernel.org, Lawrence Rust <lvr@...tsystem.co.uk>
Subject: Re: [PATCH 11/19] 8250: Fix tcsetattr to avoid ioctl(TIOCMIWAIT) hang

Hi.

This one kills the serial console for me. Luckily, in qemu:

(gdb) bt
#0  __const_udelay (xloops=4295000) at arch/x86/lib/delay.c:117
#1  0xc137ab7c in panic (fmt=0xc147f8e1 "Attempted to kill the idle task!")
    at kernel/panic.c:151
#2  0xc1020731 in do_exit (code=9) at kernel/exit.c:915
#3  0xc100476a in oops_end (flags=70, regs=<value optimized out>, signr=9)
    at arch/x86/kernel/dumpstack.c:265
#4  0xc1014317 in no_context (regs=0xc14ebe68, error_code=<value
optimized out>,
    address=192) at arch/x86/mm/fault.c:673
#5  0xc1014411 in __bad_area_nosemaphore (regs=0xc14ebe68,
    error_code=<value optimized out>, address=192, si_code=196609)
    at arch/x86/mm/fault.c:739
#6  0xc1014426 in bad_area_nosemaphore (regs=0x418958, error_code=81,
    address=3238124885) at arch/x86/mm/fault.c:746
#7  0xc101475a in do_page_fault (regs=0xc14ebe68, error_code=0)
    at arch/x86/mm/fault.c:1070
#8  0xc137ced9 in ?? () at arch/x86/kernel/entry_32.S:1265
#9  0xc11db236 in list_empty (head=0xc0) at include/linux/list.h:182
#10 0xc11db374 in waitqueue_active (q=0x98) at include/linux/wait.h:115
#11 0xc11dea4e in serial8250_do_set_termios (port=0xc1c31320,
termios=0xc14ebf2c,
    old=0xc1c31240) at drivers/serial/8250.c:2349
#12 0xc11decb6 in serial8250_set_termios (port=0xc1c31320, termios=0xc14ebf2c,
    old=0xc1c31240) at drivers/serial/8250.c:2424
#13 0xc11da10b in uart_set_options (port=0xc1c31320, co=0xc164f0a0, baud=9600,
    parity=110, bits=8, flow=110) at drivers/serial/serial_core.c:1935
#14 0xc1684d30 in serial8250_console_setup (co=0xc164f0a0, options=0x0)
    at drivers/serial/8250.c:2864
#15 0xc101e46f in register_console (newcon=0xc164f0a0) at kernel/printk.c:1300
#16 0xc1684d5e in serial8250_console_init () at drivers/serial/8250.c:2889
#17 0xc1683da0 in console_init () at drivers/tty/tty_io.c:3208
#18 0xc166b7ba in start_kernel () at init/main.c:635
#19 0xc166b0b7 in i386_start_kernel () at arch/x86/kernel/head32.c:75
#20 0x00000000 in ?? ()
(gdb) p up->port.state
$13 = (struct uart_state *) 0x0



On Sat, Nov 13, 2010 at 00:40, Greg Kroah-Hartman <gregkh@...e.de> wrote:
> From: Lawrence Rust <lvr@...tsystem.co.uk>
>
> Calling tcsetattr prevents any thread(s) currently suspended in ioctl
> TIOCMIWAIT for the same device from ever resuming.
>
> If a thread is suspended inside a call to ioctl TIOCMIWAIT, waiting for
> a modem status change, then the 8250 driver enables modem status
> interrupts (MSI).  The device interrupt service routine resumes the
> suspended thread(s) on the next MSI.
>
> If while the thread(s) are suspended, another thread calls tcsetattr
> then the 8250 driver disables MSI (unless CTS/RTS handshaking is
> enabled) thus preventing the suspended thread(s) from ever being
> resumed.
>
> This patch only disables MSI in tcsetattr handling if there are no
> suspended threads.
>
> Program to demonstrate bug & fix:
>
> /* gcc miwait.c -o miwait -l pthread */
> #include <stdio.h>
> #include <errno.h>
> #include <unistd.h>
> #include <fcntl.h>
> #include <pthread.h>
> #include <termios.h>
> #include <sys/ioctl.h>
> #include <linux/serial.h>
>
> static void* monitor( void* pv);
> static int s_fd;
>
> int main( void)
>  {
>  const char kszDev[] = "/dev/ttyS0";
>  pthread_t t;
>  struct termios tio;
>
>  s_fd = open( kszDev, O_RDWR | O_NONBLOCK);
>  if ( s_fd < 0)
>    return fprintf( stderr, "Error(%d) opening %s: %s\n", errno, kszDev, strerror( errno)), 1;
>
>  pthread_create( &t, NULL, &monitor, NULL);
>
>  /* Modem status changes seen here */
>  puts( "Main: awaiting status changes");
>  sleep( 5);
>
>  tcgetattr( s_fd, &tio);
>  tio.c_cflag ^= CSTOPB;
>
>  /* But not after here */
>  puts( "Main: tcsetattr called");
>  tcsetattr( s_fd, TCSANOW, &tio);
>
>  for (;;)
>    sleep( 1);
>  }
>
> static void* monitor( void* pv)
>  {
>  (void)pv;
>  for(;;)
>    {
>    unsigned uModem;
>    struct serial_icounter_struct cnt;
>
>    if ( ioctl( s_fd, TIOCMGET, &uModem) < 0)
>      fprintf( stderr, "Error(%d) in TIOCMGET: %s\n", errno, strerror( errno));
>    printf( "Modem status:%s%s%s%s%s%s\n",
>      (uModem & TIOCM_RTS) ? " RTS" : "",
>      (uModem & TIOCM_DTR) ? " DTR" : "",
>      (uModem & TIOCM_CTS) ? " CTS" : "",
>      (uModem & TIOCM_DSR) ? " DSR" : "",
>      (uModem & TIOCM_CD) ? " CD" : "",
>      (uModem & TIOCM_RI) ? " RI" : ""
>    );
>
>    if ( ioctl( s_fd, TIOCGICOUNT, &cnt) < 0)
>      fprintf( stderr, "Error(%d) in TIOCGICOUNT: %s\n", errno, strerror( errno));
>    printf( "Irqs: CTS:%d DSR:%d RNG:%d DCD:%d Rx:%d Tx:%d Frame:%d Orun:%d Par:%d Brk:%d Oflow:%d\n",
>      cnt.cts, cnt.dsr, cnt.rng, cnt.dcd,
>      cnt.rx, cnt.tx, cnt.frame, cnt.overrun, cnt.parity,
>      cnt.brk, cnt.buf_overrun
>    );
>
>    fputs( "Waiting...", stdout), fflush( stdout);
>    if ( 0 > ioctl( s_fd, TIOCMIWAIT, (unsigned long)(TIOCM_CAR | TIOCM_RNG | TIOCM_DSR | TIOCM_CTS)))
>      fprintf( stderr, "\nError(%d) in TIOCMIWAIT: %s\n", errno, strerror( errno));
>    fputs( "\n", stdout);
>    }
>  return NULL;
>  }
>
> Signed-off by Lawrence Rust <lawrence@...tsystem.co.uk>
>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
> ---
>  drivers/serial/8250.c |    5 ++++-
>  1 files changed, 4 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
> index 4d8e14b..dd5e1ac 100644
> --- a/drivers/serial/8250.c
> +++ b/drivers/serial/8250.c
> @@ -2343,8 +2343,11 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
>
>        /*
>         * CTS flow control flag and modem status interrupts
> +        * Only disable MSI if no threads are waiting in
> +        * serial_core::uart_wait_modem_status
>         */
> -       up->ier &= ~UART_IER_MSI;
> +       if (!waitqueue_active(&up->port.state->port.delta_msr_wait))
> +               up->ier &= ~UART_IER_MSI;
>        if (!(up->bugs & UART_BUG_NOMSR) &&
>                        UART_ENABLE_MS(&up->port, termios->c_cflag))
>                up->ier |= UART_IER_MSI;
> --
> 1.7.1
>
> --
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