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Subject[PATCH v3 00/23] ldisc fixes
Date
Sorry for the long delay. Yearly gnome desktop upgrade with lots of
new bugs, holidays, etc.

Alan, I included you in this series because you know more than most about
how the ldisc layer works. I apologize if you find this unwelcome.

Greg, I'd like special maintainer dispension for using printk(KERN_DEBUG...) in
tty: Bracket ldisc release with TTY_DEBUG_HANGUP messages
tty: Add ldisc hangup debug messages
I think this usage is in keeping with existing form (in any event, one of
messages is simply being relocated).

Changes in v3:

- Because of new changes to the way drivers interact with tty flip
buffers, this series no longer purports to prevent the flush_to_ldisc()
warning.

That said, this series does fix the warnings generated by trinity and the
various test jigs run without complaint. Ilya's latest stress_test_tty.c
runs to completion (with the read and write threads enabled) with no
WARNs (or BUGs with the series "tty: Fix parallel master and slave pty opens"
-- which just got pushed to -tty-next)

- Since the lock correction initially reported on by
Sebastian Andrzej Siewior <bigeasy@linutronix.de> here
https://lkml.org/lkml/2012/11/21/267 was fixed by Sebastian in
v3 of "tty: don't deadlock while flushing workqueue" and was
pushed to -next, the patches
"tty: Don't reenable already enable ldisc"
"tty: Don't restart ldisc on hangup if racing ldisc kill"
"tty: Make core responsible for synchronizing its work"
fix the other races which may occur when async hangup races
with the tty final close.

Please re-test with your dummy_hcd/g_nokia testcase, although
I'm not convinced that usb gadget is using tty_hangup() appropriately.
tty drivers use this for async carrier loss coming from an IRQ
which will be disabled if the tty has been shutdown. Does gserial
prevent async hangup to a dead tty in a similar fashion?

- Jiri's debug patch was removed to avoid grumblings about using
IS_ERR_OR_NULL() --
see http://lkml.indiana.edu/hypermail/linux/kernel/1301.1/00913.html

- "tty: WARN if buffer work racing with tty free" was removed
because this happens all the time now that drivers can push
i/o to a dead tty.

- The false-positive diagnostic reported by Sasha Levin here
http://lkml.indiana.edu/hypermail/linux/kernel/1212.2/01017.html
was fixed in:
"n_tty: Fully initialize ldisc before restarting buffer work"

- Fixed various comments which were either wrong or hadn't followed
the code they belonged to (but only after endless hours of
auditing various ldisc code paths)

- Added more debug log messages for use with debugging tty hangup

Changes in v2:

- Please review "tty: Don't flush buffer when closing ldisc".
This patch replaces the earlier
"tty: Don't reschedule buffer work while closing". The text of
this commit details why not calling n_tty_flush_buffer() is the
correct thing to do, so I won't repeat it here.

- The test jig has been included in the commit message for
"tty: Don't flush buffer when closing ldisc" as Alan requested.

- Ilya Zykov was added as the Signed-off-by: for the test jig in
that same commit message.

- Sasha Levin was added as the Reported-by: in that same patch.


This patch series addresses various causes of flip buffer work
being scheduled for a dead ldisc & tty.

The most common cause stems from the n_tty_close() path
flushing (which schedules buffer work), when the ldisc has already
been halted (meaning userspace has been locked out of further i/o
and existing users have finished). This is fixed in
'tty: Don't flush buffer when closing ldisc'

The other causes have a central theme: incorrect order-of-operations
when halting a line discipline. In general, to prevent buffer work
from being scheduled requires:
1. Disallowing further ldisc references
2. Waiting for all existing ldisc references to be released
3. Cancelling existing buffer work
If the wait takes place _after_ cancellation, then new work
can be scheduled by existing holder(s) of ldisc reference(s). That's
bad.

Halting the line discipline is performed when,
- hanging up the tty (tty_ldisc_hangup())
- TIOCSETD ioctl (tty_set_ldisc())
- finally closing the tty (pair) (tty_ldisc_release())

Concurrent halts are governed by the following requirements:
1. tty_ldisc_release is not concurrent with the other two and so
does not need lock or state protection during the ldiscs halt.
2. Accesses through tty->ldisc must be protected by the ldisc_mutex.
The wait operation checks the user count (ldisc references)
in tty->ldisc->users.
3. tty_set_ldisc() reschedules buffer work that was pending when
the ldiscs were halted. That must be an atomic operation in
conjunction with re-enabling the ldisc -- which necessitates
locking concurrent halts (tty_ldisc_release is exempt here)
4. The legacy mutex cannot be held while waiting for ldisc
reference(s) release -or- for cancelling buffer work.
5. Because of #4, the legacy mutex must be dropped prior to or
during the halt. Which means reacquiring after the halt. But
to preserve lock order the ldisc_mutex must be dropped and
reacquired after reacquiring the legacy mutex.
6. Because of #5, the ldisc state may have changed while the
ldisc mutex was dropped.


Peter Hurley (23):
tty: Add diagnostic for halted line discipline
n_tty: Factor packet mode status change for reuse
n_tty: Don't flush buffer when closing ldisc
tty: Refactor wait for ldisc refs out of tty_ldisc_hangup()
tty: Remove unnecessary re-test of ldisc ref count
tty: Fix ldisc halt sequence on hangup
tty: Strengthen no-subsequent-use guarantee of tty_ldisc_halt()
tty: Halt both ldiscs concurrently
tty: Remove unnecessary buffer work flush
tty: Wait for SAK work before waiting for hangup work
n_tty: Correct unthrottle-with-buffer-flush comments
tty: Kick waiters _after_ the ldisc is locked
n_tty: Fully initialize ldisc before restarting buffer work
tty: Don't reenable already enabled ldisc
tty: Don't restart ldisc on hangup if racing ldisc kill
tty: Make core responsible for synchronizing its work
tty: Document lock requirements to halt ldisc
tty: Remove stale comment re: tty_ldisc_flush_works()
tty: Fix 'deferred reopen' ldisc comment
tty: Remove stale comment re: locking in tty_ldisc_release()
tty: Re-parent orphaned tty_set_ldisc() comments
tty: Bracket ldisc release with TTY_DEBUG_HANGUP messages
tty: Add ldisc hangup debug messages

drivers/tty/n_tty.c | 62 +++++++-----
drivers/tty/tty_io.c | 23 ++++-
drivers/tty/tty_ldisc.c | 248 ++++++++++++++++++++++++++++--------------------
include/linux/tty.h | 1 +
4 files changed, 205 insertions(+), 129 deletions(-)

--
1.8.1.2



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