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Date: Mon, 12 Mar 2001 19:37:54 +0000
From: Adrian Cox <adrian@humboldt.co.uk>
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To: Jamie Lokier <lk@tantalophile.demon.co.uk>
CC: linux-kernel@vger.kernel.org
Subject: Re: static scheduling - SCHED_IDLE?
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Jamie Lokier wrote:

> Adrian Cox wrote:

>> Jamie Lokier's suggestion of raising priority when in the kernel doesn't 
>> help. You need to raise the priority of the task which is currently in 
>> userspace and will call up() next time it enters the kernel. You don't 
>> know which task that is.

> Dear oh dear.  I was under the impression that kernel semaphores are
> supposed to be used as mutexes only -- there are other mechanisms for
> signalling between processes.

I think most of the kernel semaphores are used as mutexes, with 
occasional producer/consumer semaphores. I think the core kernel code is 
fine, the risk mostly comes from miscellaneous character devices. I've 
written code that does this for a specialised device driver. I wanted 
only one process to have the device open at once, and for others to 
block on open. Using semaphores meant that multiple shells could do "cat 
 > /dev/mywidget" and be serialised.

Locking up users of this strange piece of hardware doesn't bring down 
the system, so your suggestion could work. We need a big fat warning in 
semaphore.h, and a careful examination of the current code.

- Adrian

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