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SubjectRe: [PATCH 02/10] printk: Try harder to get logbuf_lock on NMI
On Fri, 29 May 2015 12:56:07 +0200 Petr Mladek <pmladek@suse.cz> wrote:

> > > + /* try hard to get the lock but do not wait forever */
> > > + start_time = cpu_clock(this_cpu);
> > > + current_time = start_time;
> > > + while (current_time - start_time < TRY_LOCKBUF_LOCK_MAX_DELAY_NS) {
> > > + if (raw_spin_trylock(&logbuf_lock))
> > > + return 1;
> > > + cpu_relax();
> > > + current_time = cpu_clock(this_cpu);
> > > + }
> > > +
> > > + return 0;
> > > +}
> >
> > That CPU is now going to spin around for 100us and then time out.
>
> Yes, there was a deadlock without the patch. So, limited spinning is
> still a win.
>
> Or would you like to detect the deadlock immediately in all cases?
> I mean to add the proposed wrapper around take/release lock calls
> and set/test some cpu-specific variable there?

Yes. Pointlessly spinning in NMI for 100us is bad.

> It sounds interesting. Well, the detection will not be 100% correct
> because there is a small race window between taking @logbuf_lock
> and setting @lockbuf_cpu. I wonder if it is worth doing. But I will
> do it if you want.

You might be able to do something with checking logbuf_cpu within the
loop to avoid the worst-case scenarios.



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