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Date: Sun, 21 Dec 2008 13:46:28 +0800
From: David Xu <davidxu@freebsd.org>
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To: Ravi Murty <ravi.murty@gmail.com>
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Cc: freebsd-hackers@freebsd.org
Subject: Re: td_critnest
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Ravi Murty wrote:
> Hello All,
> 
> The implementation of critical_enter and critical_exit changed between
> freebsd 5 and freebsd 6. In the newer implemtnation, the code checks if
> td_critnest is 1 and if it is sets it to zero, then checks if the thread
> owes a preempt. If so, it increments td_critnest by 1 before grabbing a lock
> and then decrements it back to zero. I can't figure out why it does this.
> The freebsd 5 implementation seems straightforward where we check if the
> thread owes a preempt and if so we switch to the new thread. Can anyone help
> me with this?
> 
> Thanks
> Ravi

I guess this becauses thread_lock() also calls critical_exit(), this
code avoids recursion.