Messages in this thread | | | Date | Wed, 15 Jul 2009 22:23:08 +0530 | From | "Aneesh Kumar K.V" <> | Subject | Re: tty related hangs with 2.6.31-rc3 |
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On Wed, Jul 15, 2009 at 04:11:42PM +0100, Alan Cox wrote: > On Wed, 15 Jul 2009 18:59:56 +0530 > "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote: > > > Hi, > > > > I am finding tty related hangs with 2.6.31-rc3. This didn't happen > > before. This happen when i close the emacs session. The /proc/<pid>/stack > > content is below > > Thanks - nice clear trace. Looks like a bug in the n_tty locking changes > from a few releases back that the pty changes are triggering. > > Basically process_echoes calls tty_put_char which if it thinks a device > queue was full and now has a bit of space will call tty_wakeup which can > call process_echoes and thus deadlock. With a physical serial device we > will even sometimes call tty_wakeup() from the serial transmit path > which is an irq path (which makes this doubly wrong as it then takes > mutexes) > > Emacs presumably uses fasync which is the trigger for this. You need the > right timing combined with the new pty behaviour combined with FASYNC to > trigger it. > > > > [<c0362c5b>] process_echoes+0x2b/0x2e0 > [which tries to take the lock we already hold (end A)] > > [<c03638cb>] n_tty_write_wakeup+0xb/0x40 > [which processes our ldisc wakeup (end A)] > > [<c0360a88>] tty_wakeup+0x58/0x70 > [which wakes up our tty (end A)] > > [<c0368347>] pty_write+0x67/0x70 > [our write method is for tty/pty pairs end A output, queued to end B] > > [<c035f1cb>] tty_put_char+0x2b/0x40 > [calls tty_put_char to write the echoed byte to end A output] > > [<c036291f>] do_output_char+0xef/0x200 > [the fake typed character is echoed back towards end B] > > [<c0362d4e>] process_echoes+0x11e/0x2e0 > [tries to process echo characters on end A] > > [<c0364292>] n_tty_receive_char+0x102/0x710 > [ receives a byte that we've faked typing to end A input] > > [<c0364ac0>] n_tty_receive_buf+0x220/0x410 > [ioctl method calls the ld->ops->receive_buf for n_tty (unsafely but that > bug is old] > > [<c036059c>] tiocsti+0x8c/0xa0 > > [<c0361aca>] tty_ioctl+0x25a/0x310 > > [<c01e52c8>] vfs_ioctl+0x28/0x80 > > [<c01e54f4>] do_vfs_ioctl+0x64/0x1c0 > > [<c01e56a3>] sys_ioctl+0x53/0x70 > > [<c0102e3c>] sysenter_do_call+0x12/0x28 > > [<ffffffff>] 0xffffffff > > Do you have the lock validator enabled and if so did it have anything > useful to report ? > > Please try the following. I suspect this is the real fix: >
With limited testing i am not seeing the hang any more. I will keep running with the patch and report back if i hit the hang again.
-aneesh
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