td_critnest

[Xin LI]

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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?

My guess is to prevent race conditions (i.e. to split the owepreempt
flag into a separate variable), since this value could be changed in an
interrupt context, and not only during the current thread context.

Cheers,
- --
Xin LI <delphij at delphij.net>	http://www.delphij.net/
FreeBSD - The Power to Serve!
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