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i386 wire_count panic finally tracked down

From: Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>
Date: Wed, 26 Sep 2012 10:08:26 -0700 (PDT)

    I may have finally tracked down the i386 wire_count panic.  I noticed
    that the pmap pointer in the last pkgbox32 crash dump doesn't seem to
    be to any process's active pmap, meaning that it is related to a process
    which had exited.

    It appears to be a race against a pmap structure being dtor'd after
    a process exit and a vm_page_protect() call on a vm_page.  In this
    situation page table pages can be removed from the pmap's VM object
    unconditonally without vm_token being held, racing against a
    vm_page_protect() occuring at the same time.

    I will commit a likely fix in the next hour.

    x86-64 does not seem to be vulnerable to this particular issue but I
    am reviewing the code.


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