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

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于 2013年03月14日 22:05, Greg KH 写道:
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.
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