Date: Thu, 25 Apr 2002 23:12:15 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: JJ Behrens <jj@nttmcl.com> Cc: freebsd-stable@FreeBSD.ORG, Erik Trulsson <ertr1013@student.uu.se>, @ns.sol.net, Joe Greco <jgreco@ns.sol.net> Subject: Re: kernel trap 9 with interrupts disabled Message-ID: <200204260612.g3Q6CFq08959@apollo.backplane.com> References: <200204251836.NAA41191@aurora.sol.net> <200204252115.g3PLF6c07119@apollo.backplane.com> <20020425153121.F3601@alicia.nttmcl.com> <20020425230656.GB10349@student.uu.se> <20020425162927.C1444@alicia.nttmcl.com>
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:>
:> To get a priority inversion like this one needs at least three tasks
:> with different priorities.
:
:Ah, yes. I stand corrected :)
:
:Thanks,
:-jj
Right, you need three tasks, but one of them has absolutely nothing to
do with the process holding the resource or the processing trying
to get the resource... it simply needs to eat cpu at a priority higher
then the process holding the resource. So while you need three
processes, the situation winds up being quite common in a heavily loaded
system and utterly trivial to reproduce.
The reason this is a major problem in -stable instead of just a minor
problem is due to the fact that the 'idle priority queue' is a totally
separate queue that was hacked together and thrown into the system
without any consideration for the consequences. The kernel's normal
priority raising mechanism in tsleep() fails utterly when faced with
a totally separate queue.
This has been a well known problem in -stable for a long time, which
is why I changed the 'idprio' to be a root-only command. It's messy to
fix in -stable which is why it hasn't been... but it -current is in much
better shape. In fact, for a while in -stable nice +20 and nice -20
processes could interfere with each other in the same way even without
using 'idprio'. Fortunately that was fixed a few years ago.
-Matt
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