Messages in this thread | | | Date | Fri, 29 May 2015 13:46:13 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 02/10] printk: Try harder to get logbuf_lock on NMI |
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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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