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SubjectRe: [PATCH 01/20 -v5] printk - dont wakeup klogd with interrupts disabled
* Steven Rostedt (rostedt@goodmis.org) wrote:
>
> On Wed, 23 Jan 2008, Daniel Walker wrote:
>
> >
> > On Wed, 2008-01-23 at 11:02 -0500, Steven Rostedt wrote:
> >
> > > + if (!irqs_disabled() && wake_klogd)
> > > wake_up_klogd();
> >
> > This causes a regression .. When printk is called during an OOPS in
> > kernels without this change then the OOPS will get logged, since the
> > logging process (klogd) is woken to handle the messages.. If you apply
> > this change klogd doesn't wakeup, and hence doesn't log the oops.. So if
> > you remove the wakeup here you have to add it someplace else to maintain
> > the logging ..
> >
> > (I'm not theorizing here, I have defects logged against this specific
> > piece of code..)
>
> It wont get woken up anyway. Did you look at wake_up_klogd?
>
> void wake_up_klogd(void)
> {
> if (!oops_in_progress && waitqueue_active(&log_wait))
> wake_up_interruptible(&log_wait);
> }
>
>
> So if oops_in_progress is set, then it still wont get woken. Perhaps it
> got woken some other way? Or is oops_in_progress not set in these oops?
>
> One other solution is to make the runqueue locks visible externally. Like:
>
> in sched.c:
>
> int runqueue_is_locked(void)
> {
> int cpu = get_cpu();
> struct rq *rq = cpu_rq(cpu);
> int ret;
>
> ret = spin_is_locked(&rq->lock);
> put_cpu();
> return ret;
> }
>
> And in printk we could do:
>
> if (wake_klogd && !runqueue_is_locked())
> wake_up_klogd();
>
> This probably is the cleanest solution since it simply prevents the
> deadlock from occurring.
>


FYI :

kernel/panic.c

NORET_TYPE void panic(const char * fmt, ...)
{
....
bust_spinlocks(1);
...
bust_spinlocks(0);
...
In bust_spinlocks :


void __attribute__((weak)) bust_spinlocks(int yes)
{
if (yes) {
++oops_in_progress;
} else {
#ifdef CONFIG_VT
unblank_screen();
#endif
if (--oops_in_progress == 0)
wake_up_klogd();
}
}
Where the final wake_up happens while oops_in_progress is 0, but
interrupts are still disabled.

So about my previous email, proposing testing oops_in_progress, it just
won't work.

Mathieu

> -- Steve
>

--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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