Re: 2.6.9: serial_core: uart_open

karl malbrain
2005-07-15T20:48:12Z

----- Original Message -----
From: "Russell King" <rmk+lkml@arm.linux.org.uk>
To: "karl malbrain" <karl@petzent.com>
Cc: "Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>
Sent: Friday, July 15, 2005 1:32 PM
Subject: Re: 2.6.9: serial_core: uart_open

> On Fri, Jul 15, 2005 at 09:02:48AM -0700, karl malbrain wrote:
> > > -----Original Message-----
> > > From: Russell King
> > > Sent: Friday, July 15, 2005 12:29 AM
> > > To: karl malbrain
> > > Cc: Linux-Kernel@Vger. Kernel. Org
> > > Subject: Re: 2.6.9: serial_core: uart_open
> > >
> > >
> > > On Thu, Jul 14, 2005 at 03:35:07PM -0700, karl malbrain wrote:
> > > > AT LAST I HAVE SOME DATA!!!
> > > >
> > > > The problem is that ALL SYSTEM CALLS to open "/dev/tty" are
> > > blocking!! even
> > > > with O_NDELAY set and even from completely disjoint sessions.
> > > I discovered
> > > > this via issuing "strace sh". That's why the new xterm windows
froze.
> > > >
> > > > The original process doing the open("/dev/ttyS1", O_RDWR) is
> > > listed in the
> > > > ps aux listing as status S+.
> > >
> > > Ok, 'S' means it's sleeping.
> > >
> > > Can you enable Magic SYSRQ, and ensure that you have a large kernel
> > > log buffer (the LOG_BUF_SHIFT configuration symbol). Ensure that
> > > /proc/sys/kernel/sysrq is 1, and re-run your test such that you have
> > > something else waiting (eg, the strace sh). Then hit Alt-SysRQ-T.
> > >
> > > You can then read the kernel messages with dmesg - you may need the
> > > -s argument to capture the entire kernel buffer.
> > >
> > > This will tell us where all processes are sleeping.
> >
> >
> > sh D 00000006 3036 5341 5252 (NOTLB)
> > d0408eb0 00000086 c01c14d7 00000006 d0408e94 000f4fa5 c0d38f81 000039a0
> > df461240 df4613cc c035ff00 00000246 d0408ecc df461240 c0300e33
> > 00000001
> > df461240 c011c856 c035ff20 c035ff20 d0408000 00000001 c035abe0
> > d0408000
> > Call Trace:
> > [<c01c14d7>] inode_has_perm+0x4c/0x54
> > [<c0300e33>] __down+0x103/0x1fe
> > [<c011c856>] default_wake_function+0x0/0xc
> > [<c0301180>] __down_failed+0x8/0xc
> > [<c021a4d0>] .text.lock.tty_io+0x87/0x10f
> > [<c016d78c>] chrdev_open+0x325/0x3b9
>
> This seems to imply that there's a lock being taken in tty_open(). The
> 2.6.9 source contains no such thing. Are you sure you're using an
> unpatched 2.6.9 kernel?
>
> > [<c016256f>] dentry_open+0xbd/0x180
> > [<c01624ac>] filp_open+0x36/0x3c
> > [<c01da502>] direct_strncpy_from_user+0x46/0x5d
> > [<c0162970>] sys_open+0x31/0x7d
> > [<c03036f3>] syscall_call+0x7/0xb
>

The system is red-hat 4.6.9-11EL. There is a patch to tty_io but it doesn't
mention locking anything. karl m