Messages in this thread | | | Date | Thu, 18 Jul 2019 20:29:54 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [PATCH 2/2] printk/panic/x86: Allow to access printk log buffer after crash_smp_send_stop() |
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On (07/18/19 14:07), Konstantin Khlebnikov wrote: > > Let me test the waters. Criticize the following idea: > > > > Can we, sort of, disconnect "supposed to be dead" CPUs from printk() > > so then we can unconditionally re-init printk() from panic-CPU? > > > > We have per-CPU printk_state; so panic-CPU can set, let's say, > > DEAD_CPUS_TELL_NO_TALES bit on all CPUs but self, and vprintk_func() > > will do nothing if DEAD_CPUS_TELL_NO_TALES bit set on particular > > CPU. Foreign CPUs are not even supposed to be alive, and smp_send_stop() > > waits for IPI acks from secondary CPUs long enough on average (need > > to check that) so if one of the CPUs is misbehaving and doesn't want > > to die (geez...) we will just "disconnect" it from printk() to minimize > > possible logbuf/console drivers interventions and then proceed with > > panic; assuming that misbehaving CPUs are actually up to something > > sane. Sometimes, you know, in some cases, those CPUs are already dead: > > either accidentally powered off, or went completely nuts and do nothing, > > etc. etc. but we still can kdump() and console_flush_on_panic(). > > Good idea. > Panic-CPU could just increment state to reroute printk into 'safe' > per-cpu buffer.
Yeah, that's better.
So we can do something like this
@@ -269,15 +269,21 @@ void printk_safe_flush_on_panic(void) * Make sure that we could access the main ring buffer. * Do not risk a double release when more CPUs are up. */ - if (raw_spin_is_locked(&logbuf_lock)) { - if (num_online_cpus() > 1) - return; + debug_locks_off(); + raw_spin_lock_init(&logbuf_lock); + /* + re-init the rest of printk() locks */ + printk_safe_flush(); +}
[..]
+void printk_switch_to_panic_mode(int panic_cpu) +{ + int cpu; + for_each_possible_cpu(cpu) { + if (cpu == panic_cpu) + continue; + per_cpu(printk_context, cpu) = 42; + } }
And call printk_switch_to_panic_mode() from panic(). And we don't need to touch arch code (it also covers the case when some new ARCH will gain NMI support).
-ss
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