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From: Julian Elischer <julian@elischer.org>
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To: "Murty, Ravi" <ravi.murty@intel.com>
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Cc: freebsd-hackers@freebsd.org
Subject: Re: Do you really "sleep" when blocked on a mutex?
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Murty, Ravi wrote:
> That's actually what I was trying to get to.
> 
> If I look at vm_daemon(), it checks to see if every thread of the
> process is running, on the runq or sleeping. If any threads fails the
> condition - and I can think of the case where a thread is blocked
> waiting for a lock - it is not a target to be swapped out. I guess this
> means that if a thread is holding a lock, it can be swapped out.

no. because if it holds a lock it must be in the run queue, running or 
waiting for another lock. no other state is permitted.


> How
> does this guarantee that the thread is not holding a kernel lock? Why
> don't they allow threads waiting for a lock (blocked threads/processes)
> to be swapped out?
> 
> Related question: how can a process/thread running on a CPU be swapped
> out, do they suspend the threads before they pull out memory from
> underneath them?

not sure, I'd have to read the code again.


> 
> Thanks
> Ravi
>