witness oddity

John Baldwin jhb at FreeBSD.org
Fri Sep 10 12:09:39 PDT 2004

On Friday 10 September 2004 02:18 pm, Andrew Gallatin wrote:
> John-Mark Gurney writes:
>  > Andrew Gallatin wrote this message on Fri, Sep 10, 2004 at 13:18 -0400:
>  > > If I call copyout() holding one of my mutexes, it will always complain
>  > > about a LOR, even if the mutex is freshly initiated:
>  >
>  > Calling copyout while holding a mutex is not allowed...  If the page
>  > isn't in memory, it could take many seconds for the page to be swapped
>  > back in during which time your mutex will continue to be held.
> Thanks.. but that's not really what I asked.
> I want to know how witness detects a particular just-created mutex as
> being in a deadlock with the vm map lock.
> Again, is it because the vm lock is an sx lock?  Is there an implicit
> rule that you can't take an sx lock while holding a mutex (just like
> you can't take Giant, or sleep?)

Yes.  An sx lock is allowed to be held across a sleep, so if you block on an 
sx lock, the owner of the lock you are waiting on might be asleep.  If that 
happens, then your thread won't be able to resume until that other thread is 
woken up and it is basically akin to sleeping with a mutex.  Also, if 
copyout() takes a page fault it can sleep, so holding a mutex across copy*() 
is a bad idea anyways.

John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org

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