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	Fri, 06 Feb 2004 10:06:16 CST
Date: Fri, 6 Feb 2004 10:06:16 +0800 (CST)
From: =?gb2312?q?popsong=20old?= <oldpopsong@yahoo.com>
To: John Baldwin <jhb@FreeBSD.org>
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cc: freebsd-current@freebsd.org
Subject: Re: three locks and lock order reversal?
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> If one thread does A then B, another thread does B then C, and a third thread
> does C then A you can deadlock if each thread gets the first lock and blocks 
> on the second lock.  Thread 1 wants B and holds A, thread 2 holds B and wants
> C, and thread 3 wants A and holds C.  Thread 3 will not giveup C until it 
> gets A.  Thread 1 holds A and won't give it up until it gets B.  Thread 2 
> holds B and won't give it up until it gets C which is held by thread 3.  
> Hence, deadlock.
> 
> -- 
> John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
> "Power Users Use the Power to Serve"  =  http://www.FreeBSD.org
>

Thank you for the explanation! I only thought of two threads. So this bring me
another question: if the machine only have 2 CPUs, does it mean that this kind
of LOR is safe, provided there is no other sleep lock between A and B, B and C,
C and A?

song

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