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To: Matt Dillon <dillon@blob.best.net>
Subject: Re: possible ffs_vget() race condition 
cc: davidg@root.com, bugs@freebsd.org
In-reply-to: Your message of "Wed, 19 Jul 1995 11:09:16 PDT."
             <199507191809.LAA07002@blob.best.net> 
Date: Wed, 19 Jul 1995 11:24:51 -0700
From: Kirk McKusick <mckusick@McKusick.COM>
Sender: bugs-owner@freebsd.org
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Date: Wed, 19 Jul 1995 11:09:16 -0700
From: Matt Dillon <dillon@blob.best.net>
To: Kirk McKusick <mckusick@McKusick.COM>
Cc: davidg@root.com, bugs@freebsd.org
Subject: Re: possible ffs_vget() race condition 

    I've got some kernel printf()'s in the two critical code areas
    along with my fix so by this time tomorrow I should know whether
    the hole is getting hit.

    Oh, there are also some pretty serious problems with the sysctl()
    stuff... mainly with all the list traversals the sysctl()'s do
    without locking and the copyout's inside the loops (which can
    block, causing the list being traverse and most specifically the
    'current' item to be ripped out from under sysctl())... I had
    to stop using pstat -T because of the problem (it would crash
    the machine every couple of hours).  Since I absolutely need ps, I wound
    up patched the proc stuff sysctl's adding a lock count to the process 
    structure which the exit/zombie code blocks on, but haven't
    posted the patches because the whole sysctl thing really needs
    a good going over to solve the problems.

					-Matt

I do not believe that sysctl is the source of your problem.
For all data structures except the vnode table, it does a vslock
on the copyout buffer before starting the list traversal. Thus
the copyout cannot block. More likely your problems are arising
because the information that pstat is trying to report is coming
from /dev/kmem rather than through sysctl. Switching it to be
accessible from sysctl should solve your problem.

	~Kirk