On 24 June 2011 15:29, Dave Anderson <anderson@redhat.com> wrote:

Yeah, although the contents tty->read_buf are hard to explain.
It gets allocated during n_tty_open() and freed during n_tty_close().
And at the beginning of n_tty_read() there's:

BUG_ON(!tty->read_buf);

and the dump-time contents show a buffer allocated:

crash> tty_struct ffff8802cbd54800
struct tty_struct { ...
magic = 21505,
driver = 0xffff88031b54ea00,
ops = 0xffffffff8130f650,
name = "pts9\000\...",
driver_data = 0xffff88029c8a9668,
icanon = 1 '\001',
read_buf = 0xffff8802cbfe6000 "",
read_head = 0,
read_tail = 0,
read_cnt = 0,
...

but it's a NULL pointer when read during the function?

Hmm, that is interesting. Assuming that we are in fact dealing with a software bug where this memory area changed recently, the only possible explanation I can see is that n_tty_close() has been called while n_tty_read() is in progress. I know that's not supposed to happen, but the implication would be that is the bug - a locking issue in the tty layer that allows them to be closed while calls are in progress. Sadly that code isn't as mature as it should be and there was a whole load of concurrency issues fixed in the late 2.6.30s; I don't know the details, but it might be one of those.

Alternatively it's regular old hardware failure and we're just looking at junk.

I think that's about as far as we can go on the information available (and also the extent to which it's relevant to this mailing list)