From owner-freebsd-smp  Sun Aug 22 21:45:59 1999
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Date: Sun, 22 Aug 1999 21:45:03 -0700 (PDT)
From: Matthew Dillon <dillon@apollo.backplane.com>
Message-Id: <199908230445.VAA01758@apollo.backplane.com>
To: Alan Cox <alc@cs.rice.edu>,
	Luoqi Chen <luoqi@watermarkgroup.com>, freebsd-smp@FreeBSD.ORG
Subject: cripes, found it (was Re: Weird infinite lockup in splx())
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    Cripes.  I think it's a tight loop between two or more high priority 
    system processes.

    Here's what is going on:

    kmem_alloc_wait() locks a vm_map, tries to find space, and if it
    fails it unlocks the map and then tsleep's on the map.

    What happens when you have two processes doing this?  You would think
    it would be ok.

    But it isn't.  Because the very first field of the vm_map structure
    is a lock structure.

    The locking functions are using the *SAME* wait address as the 
    kmem_alloc_wait and kmem_free_wakeup code!

    Since we can get the locks just fine, if you have more then one process
    in this section of code the N processes rotate between each other due to
    the vm_map_unlock() calls waking up the tsleep() in the other process's
    kmem_alloc_wait.

    !!!!

    Oops.  Ok, but how to fix?  I am going to try to simply switch the first
    two fields in the vm_map structure so the lock does not sleep on the 
    same address as kmem_alloc_wait.  I believe this will fix the problem.

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>


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