Re: 2.6.9: serial_core: uart_open

[Russell King <rmk+lkml@arm.linux.org.uk>]

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

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core