On 12/04/12 20:47, Frank Rowand wrote:

The RCU stall warning functions call trigger_all_cpu_backtrace()
to print a backtrace on each cpu.  This function is only
implemented for x86.  Add a version for ARM.

With CONFIG_PREEMPT_RT_FULL enabled, flushing the output from
printk() is inhibited in some contexts to avoid increasing
real time latencies.  The RCU stall warnings are inhibited
on ARM due to this feature.  (I have not tested whether this
is also the case on other architectures.)  Add back the
oops_in_progress flag to allow the RCU stall warnings to
print immediately.

When I first implemented these patches on a locally modified
3.0.27-rt67, the call to "bust_spinlocks(0)" that I added to
print_cpu_stall() led to LOCKDEP warning of inconsistent
lock state from a trylock in serial_omap_console_write():

        else if (oops_in_progress)
                locked = spin_trylock_irqsave(&up->port.lock, flags);

If I understand correctly, the warning is triggered on the
slow lock path.  Some more of the warning is:

   inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.
   swapper/1/0 [HC1[1]:SC0[0]:HE0:SE1] takes:
    (&(&(&port->lock)->lock)->wait_lock){?.+...}, at: [<c0478ed0>] rt_spin_trylock_irqsave+0x24/0xe0

   ...

   other info that might help us debug this:
    Possible unsafe locking scenario:

          CPU0
          ----
     lock(&(&(&port->lock)->lock)->wait_lock);
     <Interrupt>
       lock(&(&(&port->lock)->lock)->wait_lock);


I have not been able to trigger the warning on recent versions
of the patches, but I suspect the latent problem might still
exist.

-Frank