Messages in this thread | | | From | Sergey Senozhatsky <> | Date | Sat, 27 Apr 2019 01:43:02 +0900 | Subject | Re: [PATCH v4] panic: add an option to replay all the printk message in buffer |
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On (04/26/19 16:14), Petr Mladek wrote: > > Then I wonder why, for example, native_stop_other_cpus() waits > 10ms at maximum after sending the NMIs. What is the state > of the CPUs that miss this deadline?
Well, I saw a case when CPU was forcibly powered off (embedded), for instance.
> > - But, more importantly, if that CPUB is in atomic context, then panic > > CPUA will spin, waiting for that CPUB to handoff printing, before > > panic CPU will even try to stop all CPUs. > > > > pr_emerg("Kernel panic - not syncing: %s\n", buf) > > > > is the point of 'synchronization' - panic CPU will wait for > > current console owner. > > "Synchronization point" is too strong formulation.
But it is sort of synchronized. That's why console_owner patch set solved the panic-printk deadlock which Google folks reported a while ago.
> The console waiter logic is effective but it does not always > work. The current console owner must be calling the console > drivers. > > > Hmm, we might have a bit of a problem here, maybe. > > Hmm, the printk() might wait forever when NMI stopped > the current console owner in the console driver code > or with the logbuf_lock taken.
I guess this is why we re-init logbuf lock from panic, however, we don't do anything with the console_owner.
> The console waiter logic might get solved by clearing > the console_owner in console_flush_on_panic(). It can't > be much worse, we already ignore console_lock() there, ...
Right.
[..] > Anyway, do we really need to have length discussion about > whether the locks are needed? They will not break anything.
I'm not objecting v5 nor your request to add that locking there. I'm talking about different things.
-ss
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