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Date: Tue, 30 Jul 2002 10:44:57 -0700
From: William Lee Irwin III <wli@holomorphy.com>
To: "Van Maren, Kevin" <kevin.vanmaren@unisys.com>
Cc: "'Andi Kleen'" <ak@suse.de>, linux-kernel@vger.kernel.org
Subject: Re: [Linux-ia64] Linux kernel deadlock caused by spinlock bug
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On Tue, Jul 30, 2002 at 12:06:54PM -0500, Van Maren, Kevin wrote:
> It isn't obvious to me how to extend those queued to reader/writer
> locks if you have to allow recursive readers without incurring the
> same overhead of tracking which processors already have a reader lock.
> If you do want to trigger recursive rw_locks, simply change the header
> file to make them normal spinlocks.  Then whenever the kernel hangs,
> see where it is. Of course, this approach only finds all of them if
> you execute every code path.
> Does anyone want to chip in on why we need recursive r/w locks?  Or why it
> is hard to remove them?  It doesn't sound like they are used much.

The tasklist_lock is taken in interrupt context by sigio generation,
and read_locks on it are permitted to be interrupted by other read_locks,
where write_locks of it must mask interrupts locally to prevent deadlock.
I think IA64 performance monitor code does it in interrupt context too.

Older (2.4.x and 2.5.x-early) took the tasklist_lock in interrupt
context to compute the load average by traversing the list of all tasks.
My concern when I changed that was largely timeslice overrun.


Cheers,
Bill
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