Re: [BUG] long freezes on thinkpad t60

[Chuck Ebbert <cebbert@redhat.com>]

On 06/14/2007 12:04 PM, Miklos Szeredi wrote:
> I've got some more info about this bug.  It is gathered with
> nmi_watchdog=2 and a modified nmi_watchdog_tick(), which instead of
> calling die_nmi() just prints a line and calls show_registers().
> 
> This makes the machine actually survive the NMI tracing.  The attached
> traces are gathered over about an hour of stressing.  An mp3 player is
> also going on continually, and I can hear a couple of seconds of
> "looping" quite often, but it gets as far as the NMI trace only
> rarely.  AFAICS only the last pair shows a trace for both CPUs during
> the same "freeze".
> 
> I've put some effort into understanding what's going on, but I'm not
> familiar with how interrupts work and that sort of thing.
> 
> The pattern that emerges is that on CPU0 we have an interrupt, which
> is trying to acquire the rq lock, but can't.
> 
> On CPU1 we have strace which is doing wait_task_inactive(), which sort
> of spins acquiring and releasing the rq lock.  I've checked some of
> the traces and it is just before acquiring the rq lock, or just after
> releasing it, but is not actually holding it.
> 
> So is it possible that wait_task_inactive() could be starving the
> other waiters of the rq spinlock?  Any ideas?

Spinlocks aren't fair, so this kind of problem is always a possibility.
I think maybe we need another kind of unlock that gives another processor
a fair chance at the lock. Some things you could try to see if they help:

- add smp_mb() after the unlock
- replace cpu_relax() with usleep()
- use an xchcg instruction to do the unlock, like i386 does when
  CONFIG_X86_OOSTORE is set