Re: open() on /dev/tty takes 30 seconds on 2.6.36

From: Eli Billauer
Date: Sat Oct 30 2010 - 20:20:16 EST

Andrew Morton wrote:

Odd. Presumably someone else was holding big_tty_mutex for 15 seconds.

We can find out who, with the sysrq-d command if you have the time
please. You'll need to enable lockdep and magic sysrq in .config. Then run `dmesg -n 8' so all messages get printed by the kernel and
then, in the middle of that 15-second delay, do

echo d > /proc/sysrq-trigger

I'll confess that I've never used sysrq-d and am unsure what the output
looks like. Fingers crossed!

First, I'll explain what I did: Since I can't login while this problem occurs, I had to modify sshd's service script to start sshd with strace, and also kick off a script (called who-is-locking.sh), which creates that sysrq call every five seconds. This could also be the place to mention, that I got an NULL dereference oops during one of these (IP at get_usage_chars+0x0/0x10e, while in the who-is-locking process). But that happened after the desired data was logged.

Now, let's look at the interesting point of the strace (of sshd):

01:23:37.156475 open("/dev/tty", O_RDWR|O_NOCTTY) = -1 ENXIO (No such device or address) <29.999444>
01:24:07.156354 setsid() = 2336 <0.000167>

The same open() taking 30 seconds, ending at 01:24:07. In /var/log/messages (below) we have
Oct 31 01:24:07 ocho modem-manager: (ttyS2) opening serial device...
Oct 31 01:24:07 ocho modem-manager: (ttyS3) closing serial device...

The same things happens again, when ssh opens /dev/pts/0, which takes 30 seconds again, with the end of operation coinciding with modem-manager opening a tty device.

As far as I know, there is only one real serial port on my computer. Maybe this snippet from my boot log may be of interest:

Oct 31 01:14:28 ocho kernel: Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
Oct 31 01:14:28 ocho kernel: serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
Oct 31 01:14:28 ocho kernel: 00:07: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
Oct 31 01:14:28 ocho kernel: serial 0000:05:04.0: PCI INT A -> GSI 16 (level, low) -> IRQ 16
Oct 31 01:14:28 ocho kernel: 0000:05:04.0: ttyS1 at I/O 0xbe20 (irq = 16) is a 8250
Oct 31 01:14:28 ocho kernel: 0000:05:04.0: ttyS2 at I/O 0xbe28 (irq = 16) is a 8250
Oct 31 01:14:28 ocho kernel: 0000:05:04.0: ttyS3 at I/O 0xbe40 (irq = 16) is a 8250
Oct 31 01:14:28 ocho kernel: Couldn't register serial port 0000:05:04.0: -28

Below is /var/log/message dump for the relevant time interval. I can submit more if necessary, but I think the picture is pretty clear.

Hope this helps,
Eli

Oct 31 01:23:59 ocho kernel: =============================================
Oct 31 01:23:59 ocho kernel:
Oct 31 01:24:04 ocho kernel: SysRq : Show Locks Held
Oct 31 01:24:04 ocho kernel:
Oct 31 01:24:04 ocho kernel: Showing all locks held in the system:
Oct 31 01:24:04 ocho kernel: 2 locks held by modem-manager/1835:
Oct 31 01:24:04 ocho kernel: #0: (big_tty_mutex){+.+.+.}, at: [<ffffffff814b9c09>] tty_lock+0x3f/0x4a
Oct 31 01:24:04 ocho kernel: #1: (&port->mutex){+.+.+.}, at: [<ffffffff812fcace>] uart_close+0x82/0x312
Oct 31 01:24:04 ocho kernel: 2 locks held by who-is-locking./1850:
Oct 31 01:24:04 ocho kernel: #0: (sysrq_key_table_lock){......}, at: [<ffffffff812e97de>] __handle_sysrq+0x28/0x15b
Oct 31 01:24:04 ocho kernel: #1: (tasklist_lock){.?.+..}, at: [<ffffffff810798a5>] debug_show_all_locks+0x4c/0x178
Oct 31 01:24:04 ocho kernel: 1 lock held by gpm/1971:
Oct 31 01:24:04 ocho kernel: #0: (tty_mutex){+.+.+.}, at: [<ffffffff812d5903>] tty_open+0x5a/0x4ac
Oct 31 01:24:04 ocho kernel: 1 lock held by mingetty/2125:
Oct 31 01:24:04 ocho kernel: #0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff812d7648>] n_tty_read+0x281/0x7b4
Oct 31 01:24:04 ocho kernel: 1 lock held by mingetty/2126:
Oct 31 01:24:04 ocho kernel: #0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff812d7648>] n_tty_read+0x281/0x7b4
Oct 31 01:24:04 ocho kernel: 1 lock held by mingetty/2127:
Oct 31 01:24:04 ocho kernel: #0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff812d7648>] n_tty_read+0x281/0x7b4
Oct 31 01:24:04 ocho kernel: 1 lock held by mingetty/2128:
Oct 31 01:24:04 ocho kernel: #0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff812d7648>] n_tty_read+0x281/0x7b4
Oct 31 01:24:04 ocho kernel: 1 lock held by mingetty/2129:
Oct 31 01:24:04 ocho kernel: #0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff812d7648>] n_tty_read+0x281/0x7b4
Oct 31 01:24:04 ocho kernel: 2 locks held by sshd/2336:
Oct 31 01:24:04 ocho kernel: #0: (tty_mutex){+.+.+.}, at: [<ffffffff812d5903>] tty_open+0x5a/0x4ac
Oct 31 01:24:04 ocho kernel: #1: (big_tty_mutex){+.+.+.}, at: [<ffffffff814b9c09>] tty_lock+0x3f/0x4a
Oct 31 01:24:04 ocho kernel:
Oct 31 01:24:04 ocho kernel: =============================================
Oct 31 01:24:04 ocho kernel:
Oct 31 01:24:07 ocho modem-manager: (ttyS2) opening serial device...
Oct 31 01:24:07 ocho modem-manager: (ttyS3) closing serial device...
Oct 31 01:24:09 ocho kernel: SysRq : Show Locks Held
Oct 31 01:24:09 ocho kernel:
Oct 31 01:24:09 ocho kernel: Showing all locks held in the system:
Oct 31 01:24:09 ocho kernel: 2 locks held by modem-manager/1835:
Oct 31 01:24:09 ocho kernel: #0: (big_tty_mutex){+.+.+.}, at: [<ffffffff814b9c09>] tty_lock+0x3f/0x4a
Oct 31 01:24:09 ocho kernel: #1: (&port->mutex){+.+.+.}, at: [<ffffffff812fcace>] uart_close+0x82/0x312
Oct 31 01:24:09 ocho kernel: 2 locks held by who-is-locking./1850:
Oct 31 01:24:09 ocho kernel: #0: (sysrq_key_table_lock){......}, at: [<ffffffff812e97de>] __handle_sysrq+0x28/0x15b
Oct 31 01:24:09 ocho kernel: #1: (tasklist_lock){.?.+..}, at: [<ffffffff810798a5>] debug_show_all_locks+0x4c/0x178
Oct 31 01:24:09 ocho kernel: 1 lock held by gpm/1971:
Oct 31 01:24:09 ocho kernel: #0: (tty_mutex){+.+.+.}, at: [<ffffffff812d5903>] tty_open+0x5a/0x4ac
Oct 31 01:24:09 ocho kernel: 1 lock held by mingetty/2125:
Oct 31 01:24:09 ocho kernel: #0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff812d7648>] n_tty_read+0x281/0x7b4
Oct 31 01:24:09 ocho kernel: 1 lock held by mingetty/2126:
Oct 31 01:24:09 ocho kernel: #0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff812d7648>] n_tty_read+0x281/0x7b4
Oct 31 01:24:09 ocho kernel: 1 lock held by mingetty/2127:
Oct 31 01:24:09 ocho kernel: #0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff812d7648>] n_tty_read+0x281/0x7b4
Oct 31 01:24:09 ocho kernel: 1 lock held by mingetty/2128:
Oct 31 01:24:09 ocho kernel: #0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff812d7648>] n_tty_read+0x281/0x7b4
Oct 31 01:24:09 ocho kernel: 1 lock held by mingetty/2129:
Oct 31 01:24:09 ocho kernel: #0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff812d7648>] n_tty_read+0x281/0x7b4
Oct 31 01:24:09 ocho kernel: 2 locks held by sshd/2336:
Oct 31 01:24:09 ocho kernel: #0: (tty_mutex){+.+.+.}, at: [<ffffffff812d5903>] tty_open+0x5a/0x4ac
Oct 31 01:24:09 ocho kernel: #1: (big_tty_mutex){+.+.+.}, at: [<ffffffff814b9c09>] tty_lock+0x3f/0x4a
Oct 31 01:24:09 ocho kernel:
Oct 31 01:24:09 ocho kernel: =============================================
Oct 31 01:24:09 ocho kernel:

--

Web: http://www.billauer.co.il

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/