Date: Fri, 10 Sep 2004 14:50:47 -0400 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-current@FreeBSD.org Cc: Andrew Gallatin <gallatin@cs.duke.edu> Subject: Re: witness oddity Message-ID: <200409101450.47478.jhb@FreeBSD.org> In-Reply-To: <16705.61418.553065.584034@grasshopper.cs.duke.edu> References: <16705.57806.550902.483858@grasshopper.cs.duke.edu> <20040910172515.GU72089@funkthat.com> <16705.61418.553065.584034@grasshopper.cs.duke.edu>
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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@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
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