Re: [PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in the debug code

[Byungchul Park]

On Fri, Jan 29, 2016 at 01:05:00PM +0900, Sergey Senozhatsky wrote:
> then this will explode:
> 
> printk
>  spin_lock
>   >> coding error <<
>  spin_unlock
>   printk
>    spin_lock
>     printk
>      spin_lock
>       printk
>        spin_lock
>         ... boom
> 
> vprintk_emit() recursion detection code will not work for logbuf_lock here.
> because the only criteria how vprintk_emit() can detect a recursion is via
> static `logbuf_cpu' which is set to UINT_MAX right before it
> raw_spin_unlock(&logbuf_lock). so from vprintk_emit() POV the logbuf_lock is
> already unlocked. which is not true.
> 
> 
> in case of memory corruption I don't think we must care, 'coding error case'
> is _probably/may be_ something that can be improved, but I'm not really 100%
> sure... and this still doesn't explain your console_sem.lock case.

Hello, I found the case this bad thing can happen. So the thought occurred
struck me that we need a patch, similar to my v3 patch, even though the
consideration of logbug_lock in the v3 patch may not be necessary now.

cpu0
====
printk
  console_trylock
  console_unlock
    up_console_sem
      up
        raw_spin_lock_irqsave(&sem->lock, flags)
        __up
          wake_up_process
            try_to_wake_up
              raw_spin_lock_irqsave(&p->pi_lock)
                __spin_lock_debug
                  spin_dump // once it happened
                    printk
                      console_trylock
                        raw_spin_lock_irqsave(&sem->lock, flags)

                        <=== DEADLOCK

cpu1
====
printk
  console_trylock
    raw_spin_lock_irqsave(&sem->lock, flags)
    __spin_lock_debug
      spin_dump
        printk
          ...

          <=== repeat the recursive cycle infinitely

This was the my v3 patch.
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