Date:	Sat, 13 Nov 2010 20:22:07 +0100
From:	Lawrence Rust <lawrence@...tsystem.co.uk>
To:	Greg KH <gregkh@...e.de>
Cc:	Alexey Zaytsev <alexey.zaytsev@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/19] 8250: Fix tcsetattr to avoid ioctl(TIOCMIWAIT)
 hang

On Sat, 2010-11-13 at 06:49 -0800, Greg KH wrote:
> On Sat, Nov 13, 2010 at 07:18:30AM +0300, Alexey Zaytsev wrote:
> > 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
> > 
> 
> Ick.
> 
> Lawrence, any ideas?  If not, I'm going to have to revert this patch
> from the tree.

Maybe best to pull the patch for now and have a re-think.  Shame, it
seemed so easy and the fix works for me with a simple modem input.  I'll
re-submit later..

-- Lawrence

> thanks,
> 
> greg k-h
> 
> 
> > 
> > 
> > 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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> > >


-- 
Lawrence


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