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Date: Fri, 29 Jan 2016 15:54:07 +0900
From: Byungchul Park <byungchul.park@lge.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Peter Hurley <peter@hurleysoftware.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        akpm@linux-foundation.org, mingo@kernel.org,
        linux-kernel@vger.kernel.org, akinobu.mita@gmail.com, jack@suse.cz,
        torvalds@linux-foundation.org
Subject: Re: [PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in
 the debug code
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On Fri, Jan 29, 2016 at 09:27:03AM +0900, Sergey Senozhatsky wrote:
> 
> well, the stack is surely limited, but on every
> spin_dump()->spin_lock() recursive call it does another
> round of
> 
> 	u64 loops = loops_per_jiffy * HZ;
> 
> 	for (i = 0; i < loops; i++) {
> 		if (arch_spin_trylock(&lock->raw_lock))
> 			return;
> 		__delay(1);
> 	}
> 
> so if you have 1000 spin_dump()->spin_lock() then, well,
> something has been holding the lock for '1000 * loops_per_jiffy * HZ'.

Or the printk() is heavily called and the lock is congested.