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

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Message-ID: <20160128154257.GA564@swordfish>
Date:	Fri, 29 Jan 2016 00:42:57 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Byungchul Park <byungchul.park@....com>
Cc:	akpm@...ux-foundation.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, akinobu.mita@...il.com, jack@...e.cz,
	torvalds@...ux-foundation.org, peter@...leysoftware.com,
	sergey.senozhatsky@...il.com,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in
 the debug code

On (01/28/16 19:53), Sergey Senozhatsky wrote:
> > ah... silly me... you mean the first CPU that triggers the spin_dump() will
> 					^^^ this, of course, is true for
> 					console_sem->lock and logbuf_lock
> 					only.
> 
> > deadlock itself, so the rest of CPUs will see endless recursive
> > spin_lock()->spin_dump()->spin_lock()->spin_dump() calls?
[..]
> > Can you please update your bug description in the commit message?
> > It's the deadlock that is causing the recursion on other CPUs in the
> > first place.

no, don't update anything. I was completely wrong. it's not a deadlock
that is the root cause here.

even if at some level of recursion (nested printk calls)
spin_dump()->__spin_lock_debug()->arch_spin_trylock() acquires the
lock, it returns back with the spin lock unlocked anyway.

vprintk_emit()
 console_trylock()
  spin_lock()
   spin_dump()
    vprintk_emit()
     console_trylock()
      spin_lock()
       spin_dump()
        vprintk_emit()
         console_trylock()
          spin_lock()     << OK, got the lock finally
           sem->count--
          spin_unlock()   << unlock, return
       arch_spin_lock()   << got the lock, return
      sem->count--
      spin_unlock() << unlock, return
   arch_spin_lock() << got the lock, return
  sem->count--
  spin_unlock() << unlock, return


...um


> But I found there's a possiblity in the debug code *itself* to cause a
> lockup.

please explain.

	-ss