Add a CONFIG option to allow the output from Magic SysRq to be output
immediately, even if this causes large latencies.

If PREEMPT_RT_FULL, printk() will not try to acquire the console lock
when interrupts or preemption are disabled.  If the console lock is
not acquired the printk() output will be buffered, but will not be
output immediately. Some drivers call into the Magic SysRq code
with interrupts or preemption disabled, so the output of Magic SysRq
will be buffered instead of printing immediately if this option is
not selected.

Even with this option selected, Magic SysRq output will be delayed
if the attempt to acquire the console lock fails.

Signed-off-by: Frank Rowand <frank.rowand@am.sony.com>
Link: http://lkml.kernel.org/r/4E7CEF60.5020508@am.sony.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

On 07/27/2011 04:37 PM, Thomas Gleixner wrote:
>  - KGDB (not yet disabled) is reportedly unusable on -rt right now due
>    to missing hacks in the console locking which I dropped on purpose.
>

To work around this in the short term you can use this patch, in
addition to the clocksource watchdog patch that Thomas brewed up.

Comments are welcome of course.  Ultimately the right solution is to
change separation between the console and the HW to have a polled mode
+ work queue so as not to introduce any kind of latency.

Thanks,
Jason.

Beyond the warning:

 drivers/tty/serial/8250.c:1613:6: warning: unused variable ‘pass_counter’ [-Wunused-variable]

the solution of just looping infinitely was ugly - up it to 1 million to
give it a chance to continue in some really ugly situation.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Signed-off-by: Ingo Molnar <mingo@elte.hu>

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Fixes the following on PREEMPT_RT:

BUG: sleeping function called from invalid context at kernel/rtmutex.c:684
in_atomic(): 0, irqs_disabled(): 1, pid: 9116, name: sshd
Pid: 9116, comm: sshd Not tainted 2.6.31-rc6-rt2 #6
Call Trace:
[<ffffffff81034a4f>] __might_sleep+0xec/0xee
[<ffffffff812fbc6d>] rt_spin_lock+0x34/0x75
[ffffffff81064a83>] atomic_dec_and_spin_lock+0x36/0x54
[<ffffffff811df7c7>] put_ldisc+0x57/0xa6
[<ffffffff811dfb87>] tty_ldisc_hangup+0xe7/0x19f
[<ffffffff811d9224>] do_tty_hangup+0xff/0x319
[<ffffffff811d9453>] tty_vhangup+0x15/0x17
[<ffffffff811e1263>] pty_close+0x127/0x12b
[<ffffffff811dac41>] tty_release_dev+0x1ad/0x4c0
....

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

commit dbf1115d3f8c7052788aa4e6e46abd27f3b3eeba upstream.

Patch to fix a spinlock lockup in the driver that sometimes happens when the
tasklet starts.

Signed-off-by: Claudio Scordino <claudio@evidence.eu.com>
Signed-off-by: Dave Bender <codehero@gmail.com>
Tested-by: Dave Bender <codehero@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

* 'tty-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty:
  TTY: ldisc, wait for ldisc infinitely in hangup
  TTY: ldisc, move wait idle to caller
  TTY: ldisc, allow waiting for ldisc arbitrarily long
  Revert "tty/serial: Prevent drop of DCD on suspend for Tegra UARTs"
  RS485: fix inconsistencies in the meaning of some variables
  pch_uart: Fix DMA resource leak issue
  serial,mfd: Fix CMSPAR setup
  tty/serial: Prevent drop of DCD on suspend for Tegra UARTs
  pch_uart: Change company name OKI SEMICONDUCTOR to LAPIS Semiconductor
  pch_uart: Support new device LAPIS Semiconductor ML7831 IOH
  pch_uart: Fix hw-flow control issue
  tty: hvc_dcc: Fix duplicate character inputs
  jsm: Change maintainership