Messages in this thread | | | Date | Wed, 23 Mar 2016 10:24:43 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [RFC][PATCH v6 1/2] printk: Make printk() completely async |
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On (03/22/16 17:36), Petr Mladek wrote: > > - /* cpu currently holding logbuf_lock in this function */ > > - static unsigned int logbuf_cpu = UINT_MAX; > > + bool in_panic = console_loglevel == CONSOLE_LOGLEVEL_MOTORMOUTH; > > I am just looking at the printk in NMI patchset and I will need to > deal with the panic state as well. I am not sure if this detection > is secure. > > This console level is set also by kdb_show_stack() > and kdb_dumpregs(). I am not sure how this kdb stuff works > and if it affects normal kernel but... > > Anyway, it seems that many locations detects the panic situation > via the variable oops_in_progress. It has another advantage > that it can be easily checked and we would not need any extra > variable here.
oops_in_progress is not my favorite global. and we can't rely on it in async printk.
in panic() we have
console_verbose(); bust_spinlocks(1); << sets to one
pr_emerg("Kernel panic - not syncing: %s\n", buf); smp_send_stop();
bust_spinlocks(0); << sets it back to zero
console_flush_on_panic();
there are several issues here. - first, panic_cpu does not see oops_in_progress right after bust_spinlocks(0). thus all printk issued from panic_cpu can go via async printk.
- second, smp_send_stop() does not guarantee that all of the CPUs received STOP IPI by the time it returns. on some platforms (ARM, for instance) smp_send_stop()
: if (!cpumask_empty(&mask)) : smp_cross_call(&mask, IPI_CPU_STOP); : : /* Wait up to one second for other CPUs to stop */ : timeout = USEC_PER_SEC; : while (num_online_cpus() > 1 && timeout--) : udelay(1); : : if (num_online_cpus() > 1) : pr_warn("SMP: failed to stop secondary CPUs\n"); : : return;
waits for one second and returns back to panic_cpu, and panic_cpu sets oops_in_progress back to zero. simulataneously SOPT_IPIs can start arriving to remaining CPUs. on some platforms (ARM, for instance) STOP_IPI is
: raw_spin_lock(&stop_lock); : pr_crit("CPU%u: stopping\n", cpu); : dump_stack(); : raw_spin_unlock(&stop_lock); : } : : set_cpu_online(cpu, false); : : local_fiq_disable(); : local_irq_disable(); : : while (1) : cpu_relax();
so CPUs dump_stack()s can in theory happen when oops_in_progress is zero and, thus, printk will try to print it by printk_kthread, which is not something we really want to do in panic().
so I wanted to have in printk some panic indication that once set never gets cleared. my proposal was
void console_panic(void) { printk_sync = false; }
or similar.
-ss
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