Message-ID: <3A0C90FD.CB645430@uow.edu.au>
Date: Sat, 11 Nov 2000 11:21:17 +1100
From: Andrew Morton <andrewm@uow.edu.au>
MIME-Version: 1.0
To: George Anzinger <george@mvista.com>
CC: Keith Owens <kaos@ocs.com.au>, John Kacur <jkacur@home.com>,
        linux-kernel@vger.kernel.org
Subject: Re: test11-pre2 compile error undefined reference to `bust_spinlocks' 
 WHAT?!
In-Reply-To: <23569.973832900@kao2.melbourne.sgi.com> <3A0C2D4A.83C75D4B@mvista.com>
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Sender: linux-kernel-owner@vger.kernel.org

George Anzinger wrote:
> 
> The notion of releasing a spin lock by initializing it seems IMHO, on
> the face of it, way off.  Firstly the protected area is no longer
> protected which could lead to undefined errors/ crashes and secondly,
> any future use of spinlocks to control preemption could have a lot of
> trouble with this, principally because the locker is unknown.
> 
> In the case at hand, it would seem that an unlocked path to the console
> is a more correct answer that gives the system a far better chance of
> actually remaining viable.
> 

Does bust_spinlocks() muck up the preemptive kernel's spinlock
counting?  Would you prefer spin_trylock()/spin_unlock()?
It doesn't matter - if we call bust_spinlocks() the kernel is
known to be dead meat and there is a fsck in your near future.

We are still trying to find out why kumon@fujitsu's 8-way is
crashing on the test10-pre5 sched.c.  Looks like it's fixed
in test11-pre2 but we want to know _why_ it's fixed.  And at
present each time he hits the bug, his printk() deadlocks.

So bust_spinlocks() is a RAS feature :)  A very important one -
it's terrible when your one-in-a-trillion bug happens and there
are no diagnostics.

It's a work-in-progress.  There are a lot of things which
can cause printk to deadlock:

- console_lock
- timerlist_lock
- global_irq_lock (console code does global_cli)
- log_wait.lock
- tasklist_lock (printk does wake_up) (*)
- runqueue_lock (printk does wake_up)

I'll be proposing a better patch for this in a few days.

(*) Keith: this explains why you can't do a printk() in
__wake_up_common: printk calls wake_up().  Duh.
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