N_PPP_SYNC ldisc BUG: sleeping function called from invalid context

[Tilman Schmidt <tilman@imap.cc>]

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While testing pppd with capiplugin on kernel 2.6.31 (plus my CAPI and
Gigaset patches) I came across this BUG message. (Please ignore the
Tainted flag, it's from the Nvidia driver and doesn't influence what's
following.)

BUG: sleeping function called from invalid context at kernel/mutex.c:280
in_atomic(): 1, irqs_disabled(): 1, pid: 10, name: events/0
INFO: lockdep is turned off.
irq event stamp: 2034
hardirqs last  enabled at (2033): [<c0347b07>]
_spin_unlock_irqrestore+0x3c/0x6c
hardirqs last disabled at (2034): [<c03475d0>] _spin_lock_irq+0x11/0x3f
softirqs last  enabled at (2008): [<c012f598>] __do_softirq+0x114/0x11c
softirqs last disabled at (1971): [<c010593a>] do_softirq+0x69/0xc7
Pid: 10, comm: events/0 Tainted: P           2.6.31-vanilla #1
Call Trace:
 [<c0124a3c>] __might_sleep+0x10e/0x116
 [<c0346bdd>] mutex_lock_nested+0x18/0x31
 [<c026d39b>] tty_unthrottle+0x10/0x38
 [<f8dcc31f>] ppp_sync_receive+0x168/0x170 [ppp_synctty]
 [<f8fbb9ce>] handle_minor_recv+0x187/0x1cd [capi]
 [<f8fbc19b>] capi_recv_message+0x1d9/0x24e [capi]
 [<fa5be461>] recv_handler+0x56/0x6f [kernelcapi]
 [<c0138cba>] worker_thread+0x14a/0x21d
 [<c0138c9e>] ? worker_thread+0x12e/0x21d
 [<fa5be40b>] ? recv_handler+0x0/0x6f [kernelcapi]
 [<c013b375>] ? autoremove_wake_function+0x0/0x30
 [<c013b201>] kthread+0x64/0x69
 [<c0138b70>] ? worker_thread+0x0/0x21d
 [<c013b19d>] ? kthread+0x0/0x69
 [<c0103f33>] kernel_thread_helper+0x7/0x10

Turns out the ppp_sync_receive() function (drivers/net/ppp_synctty.c
line 385ff.) has a comment in front stating:

/*
 * This can now be called from hard interrupt level as well
 * as soft interrupt level or mainline.
 */

but calls tty_unthrottle() which in turn calls mutex_lock() which of
course can sleep.

That tty_unthrottle() call was already removed once by

commit a6540f731d506d9e82444cf0020e716613d4c46c
Author: Alan Cox <alan@linux.intel.com>
Date:   Mon Jun 22 18:42:29 2009 +0100

    ppp: Fix throttling bugs

but re-added by

commit 4a21b8cb3550f19f838f7c48345fbbf6a0e8536b
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu Jul 16 09:14:23 2009 -0700

    Revert "ppp: Fix throttling bugs"

    This reverts commit a6540f731d506d9e82444cf0020e716613d4c46c, as
    requested by Alan:

      "... as it was wrong, the pty code is now fixed and the fact this
       isn't reverted is breaking pptp setups."

Opinions?

Thanks,
Tilman

- --
Tilman Schmidt                    E-Mail: tilman@imap.cc
Bonn, Germany
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