Re: [BUG]: when printk too more through serial, cpu up is failed.

[Shuge]

ä 2013å03æ14æ 22:05, Greg KH åé:
> On Thu, Mar 14, 2013 at 09:51:34PM +0800, Shuge wrote:
> > Hi all,
> >      When the kernel printk too many log, the cpu is failed to come online.
> > The problem is this:
> > For example, cpu0 bring up cpu1:
> >
> >        a. cpu0 call cpu_up:
> >           cpu_up()
> >           ->_cpu_up()
> >              ->__cpu_notify(CPU_UP_PREPARE)
> >              ->__cpu_up()
> >                 ->boot_secondary()
> > #       ->wait_for_completion_timeout(&cpu_running, msecs_to_jiffires(1000))
> >           -> if (!cpu_online(cpu)) {
> >                    pr_crit("CPU%u: failed to come online\n", cpu);
> >                    ret = -EIO;
> >                }
> >           ->cpu_notify(CPU_ONLINE)
> >
> >        b. cpu1 enter kernel:
> >        secondary_start_kernel()
> > @   ->printk("CPU%u: Booted secondary processor\n", cpu)
> > *   ->calibrate_delay()
> >        ->set_cpu_online()
> >        ->complete(cpu_running)
> >          ->cpumask_set_cpu()
> >
> >     While cpu0 run to mark #,  which wait that cpu1 complete
> > cpu_running, and set online.
> > Generally, cpu0 can get it. But if the __log_buf is too large or
> > other threads write
> > it unceasing, then cpu1 come to mark @ or * in this moment. Cpu1 is
> > busy outputing
> > buffer, which cost time more than 1s, and cpu1 have not join in
> > sched, so cpu0 wait it timeout.
> >     By reading printk.c, I found that can_use_console() always return
> > true, which be called by
> > console_trylock_for_printk(). Because, have_callable_console()
> > return ture always, if the console
> > driver set CON_ANYTIME flag. I think that cpu should not output the
> > __log_buf in coming online,
> > even though have_callable_console() is true.
> >
> > /*
> >   * Can we actually use the console at this time on this cpu?
> >   *
> >   * Console drivers may assume that per-cpu resources have
> >   * been allocated. So unless they're explicitly marked as
> >   * being able to cope (CON_ANYTIME) don't call them until
> >   * this CPU is officially up.
> >   */
> > static inline int can_use_console(unsigned int cpu)
> > {
> >      return cpu_online(cpu) || have_callable_console();
> > }
> >
> > In can_use_console, why not is &&, but ||?
> >
> > Kernel Version: 3.3.0
> Why such an old and obsolete kernel version?  Please try this on 3.8,
> lots of work have gone into the printk area that should have solved this
> issue.
>
> greg k-h

I saw the printk.c in version 3.9, it still check 
console_trylock_for_printk() to decide to call console_unlock. In 
vprintk_emit(), cpu1 also have the opportunity to execute 
console_unlock() at coming online time.
Once cpu which is coming online can output buffer, nothing can interrupt 
it until buffer is empty.But we can't ensure that none always write the 
__log_buf. It is danger! I think, the solution is that we should prevent 
to use console at coming online.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/