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  1. Oct 23, 2012
    • Ivo Sieben's avatar
      tty: prevent unnecessary work queue lock checking on flip buffer copy · cee4ad1e
      Ivo Sieben authored 
      

When low_latency flag is set the TTY receive flip buffer is copied to the
line discipline directly instead of using a work queue in the background.
Therefor only in case a workqueue is actually used for copying data to the
line discipline we'll have to flush the workqueue.

This prevents unnecessary spin lock/unlock on the workqueue spin lock that
can cause additional scheduling overhead on a PREEMPT_RT system. On a 200
MHz AT91SAM9261 processor setup this fixes about 100us of scheduling
overhead on the TTY read call.

Signed-off-by: default avatarIvo Sieben <meltedpianoman@gmail.com>
Acked-by: default avatarAlan Cox <alan@linux.intel.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
      cee4ad1e
    • Daniel Vetter's avatar
      console: implement lockdep support for console_lock · daee7797
      Daniel Vetter authored 
      Dave Airlie recently discovered a locking bug in the fbcon layer,
where a timer_del_sync (for the blinking cursor) deadlocks with the
timer itself, since both (want to) hold the console_lock:

https://lkml.org/lkml/2012/8/21/36



Unfortunately the console_lock isn't a plain mutex and hence has no
lockdep support. Which resulted in a few days wasted of tracking down
this bug (complicated by the fact that printk doesn't show anything
when the console is locked) instead of noticing the bug much earlier
with the lockdep splat.

Hence I've figured I need to fix that for the next deadlock involving
console_lock - and with kms/drm growing ever more complex locking
that'll eventually happen.

Now the console_lock has rather funky semantics, so after a quick irc
discussion with Thomas Gleixner and Dave Airlie I've quickly ditched
the original idead of switching to a real mutex (since it won't work)
and instead opted to annotate the console_lock with lockdep
information manually.

There are a few special cases:
- The console_lock state is protected by the console_sem, and usually
  grabbed/dropped at _lock/_unlock time. But the suspend/resume code
  drops the semaphore without dropping the console_lock (see
  suspend_console/resume_console). But since the same thread that did
  the suspend will do the resume, we don't need to fix up anything.

- In the printk code there's a special trylock, only used to kick off
  the logbuffer printk'ing in console_unlock. But all that happens
  while lockdep is disable (since printk does a few other evil
  tricks). So no issue there, either.

- The console_lock can also be acquired form irq context (but only
  with a trylock). lockdep already handles that.

This all leaves us with annotating the normal console_lock, _unlock
and _trylock functions.

And yes, it works - simply unloading a drm kms driver resulted in
lockdep complaining about the deadlock in fbcon_deinit:

======================================================
[ INFO: possible circular locking dependency detected ]
3.6.0-rc2+ #552 Not tainted
-------------------------------------------------------
kms-reload/3577 is trying to acquire lock:
 ((&info->queue)){+.+...}, at: [<ffffffff81058c70>] wait_on_work+0x0/0xa7

but task is already holding lock:
 (console_lock){+.+.+.}, at: [<ffffffff81264686>] bind_con_driver+0x38/0x263

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (console_lock){+.+.+.}:
       [<ffffffff81087440>] lock_acquire+0x95/0x105
       [<ffffffff81040190>] console_lock+0x59/0x5b
       [<ffffffff81209cb6>] fb_flashcursor+0x2e/0x12c
       [<ffffffff81057c3e>] process_one_work+0x1d9/0x3b4
       [<ffffffff810584a2>] worker_thread+0x1a7/0x24b
       [<ffffffff8105ca29>] kthread+0x7f/0x87
       [<ffffffff813b1204>] kernel_thread_helper+0x4/0x10

-> #0 ((&info->queue)){+.+...}:
       [<ffffffff81086cb3>] __lock_acquire+0x999/0xcf6
       [<ffffffff81087440>] lock_acquire+0x95/0x105
       [<ffffffff81058cab>] wait_on_work+0x3b/0xa7
       [<ffffffff81058dd6>] __cancel_work_timer+0xbf/0x102
       [<ffffffff81058e33>] cancel_work_sync+0xb/0xd
       [<ffffffff8120a3b3>] fbcon_deinit+0x11c/0x1dc
       [<ffffffff81264793>] bind_con_driver+0x145/0x263
       [<ffffffff81264a45>] unbind_con_driver+0x14f/0x195
       [<ffffffff8126540c>] store_bind+0x1ad/0x1c1
       [<ffffffff8127cbb7>] dev_attr_store+0x13/0x1f
       [<ffffffff8116d884>] sysfs_write_file+0xe9/0x121
       [<ffffffff811145b2>] vfs_write+0x9b/0xfd
       [<ffffffff811147b7>] sys_write+0x3e/0x6b
       [<ffffffff813b0039>] system_call_fastpath+0x16/0x1b

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(console_lock);
                               lock((&info->queue));
                               lock(console_lock);
  lock((&info->queue));

 *** DEADLOCK ***

v2: Mark the lockdep_map static, noticed by Jani Nikula.

Cc: Dave Airlie <airlied@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
      daee7797
  3. Oct 21, 2012
  5. Oct 20, 2012