linux-kernel - Re: [RFC 1/1] Char: mxser_new, fix recursive locking

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Message-ID: <20070414195543.GE20376@fi.muni.cz>
Date:	Sat, 14 Apr 2007 21:55:43 +0200
From:	Jan Yenya Kasprzak <kas@...muni.cz>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	linux-kernel@...r.kernel.org, osv@...ad.com
Subject: Re: [RFC 1/1] Char: mxser_new, fix recursive locking

Jiri Slaby wrote:
: >        I have another problem with the driver - it probably sometimes
: >drops DCD signal on the serial line or something like that:
: >when the traffic on the serial console is heavy, it sometimes disconnects
: >me from the remote shell, and cu(1) displays the login prompt from the new
: >instance of mgetty of the remote machine. However, it does so both with
: >mxser.o and mxser_new.o (in 2.6.21-rc6, I think it worked in 2.6.19,
: >but I have to retest it). So this is another problem, different from
: >the one we are trying to solve now.
: 
: There were some changes, however nothing significant in mxser.c, maybe
: some of tty or ldisc layer changes (but there is only termios ->
: ktermios switch + some other things), this would probably be hard to
: find without bisecting if 2.6.19 is really OK for you.

	OK, I'll try to bisect in a few weeks.

: The only idea I have right now is to nohup process, which will
: int fd = open("/dev/ttyMIXX", O_RDONLY | O_NONBLOCK);
: while (1) {
:  ioctl(fd, TIOCMIWAIT, TIOCM_CD);
:  ioctl(fd, TIOCMGET, &ret);
:  printf("%ld: carrier has changed: %u\n", time(NULL), !!(ret & TIOCM_CD));
: }

	Hmm, I have tried to run this, and got a machine lockup, and after
a minute or so the following has been printed to the console:

~BUG: spinlock lockup on CPU#0, sshd/1671, ffffffff80557780

(I was logged in using ssh, and ran the above code inside the ssh session).

	I have tried to look at it from the remote side as well:
stracing the "find / -print" revealed that after some time,
the write(1, "/file/name/...", ...) call returned -EIO.
And from this point on, all subsequent writes to fd #1 did return
-EIO as well.

-Yenya

-- 
| Jan "Yenya" Kasprzak  <kas at {fi.muni.cz - work | yenya.net - private}> |
| GPG: ID 1024/D3498839      Fingerprint 0D99A7FB206605D7 8B35FCDE05B18A5E |
| http://www.fi.muni.cz/~kas/    Journal: http://www.fi.muni.cz/~kas/blog/ |
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