Re: [PATCH] 2.4.16 kernel/printk.c (per processor initialization check)

[Andrew Morton]

[e-mail] wrote:
> 
> Hello,
> 
> I experienced system hang on my SMP machine and it turned out to be due to
> console write before mmu initialization completes.
> 
> To be more specific, even if secondary processors are not in status enough
> to do actual console I/O (e.g. mmu is not initialized), call_console_drivers()
> tries to do it.
> This leads to unpredictable result. For me, for example, it cause machine
> check abort and hang up system.
> 
> Attached is a patch for it.
> 
> --- kernel/printk.c     2001/11/27 04:41:49     1.1.1.8
> +++ kernel/printk.c     2001/12/03 05:25:26
> @@ -491,20 +491,22 @@
>   */
>  void release_console_sem(void)
>  {
>         unsigned long flags;
>         unsigned long _con_start, _log_end;
>         unsigned long must_wake_klogd = 0;
> 
>         for ( ; ; ) {
>                 spin_lock_irqsave(&logbuf_lock, flags);
>                 must_wake_klogd |= log_start - log_end;
> +               if (!(cpu_online_map & 1UL << smp_processor_id()))
> +                       break;
>                 if (con_start == log_end)
>                         break;                  /* Nothing to print */
>                 _con_start = con_start;
>                 _log_end = log_end;
>                 con_start = log_end;            /* Flush */
>                 spin_unlock_irqrestore(&logbuf_lock, flags);
>                 call_console_drivers(_con_start, _log_end);
>         }
>         console_may_schedule = 0;
>         up(&console_sem);
> 

Seems that there is some sort of ordering problem here - someone
is calling printk before the MMU is initialised, but after some
console drivers have been installed.

I suspect the real fix is elsewhere, but I'm not sure where.

Probably a clearer place to put this test would be within
printk itself, immediately before the down_trylock.  Does that
work?

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