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

[Andrew Morton]

On Sat, 30 Oct 2010 09:47:26 +0200 Eli Billauer <eli@xxxxxxxxxxxxxx> wrote:

> Hello,
> 
> I'm sorry about my previous overconclusive email, but there is a real 
> problem with opening TTYs during a few minutes after a system boots 
> (Fedora 12 in my case). I'll stick to the facts this time.
> 
> The kernel involved is 2.6.36, downloaded a few days ago as "latest 
> stable kernel" at kernel.org.
> 
> Running strace on sshd with -T and -tt flags, hence showing the time 
> each call took, these two lines were found. The number in brackets is 
> the time the system call took (not to the time to the next call or 
> something).
> 
> ...
> 21:35:21.131436 open("/dev/tty", O_RDWR|O_NOCTTY) = -1 ENXIO (No such 
> device or address) <30.022532>
> ...
> 21:35:51.175642 open("/dev/pts/0", O_RDWR) = 4 <30.063213>
> ...
> 
> So it took 30 seconds just to fail opening /dev/tty.
> 
> I then went on to add printk's in pty.c. Among others, I had:
> 
> static int ptmx_open(struct inode *inode, struct file *filp)
> {
>     struct tty_struct *tty;
>     int retval;
>     int index;
> 
>     nonseekable_open(inode, filp);
> 
>     /* find a device that is not in use. */
>     printk(KERN_ALERT  "34: ptmx_open to lock\n");
>     tty_lock();
>     printk(KERN_ALERT  "35: ptmx_open locked\n");
> 
> [snipped here]
> 
> And then found in my /var/log/messages (no log lines between these two):
> Oct 29 16:14:58 ocho kernel: 34: ptmx_open to lock
> Oct 29 16:15:13 ocho kernel: 35: ptmx_open locked
> 
> So we can see it took 15 seconds to acquire a lock in this case. In all 
> other pairs of lock messages there was no time difference. To me it 
> looks like 15 seconds in order to acquire a lock in the kernel is a 
> smoking gun.
> 

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!

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