From: Keith Owens <kaos@ocs.com.au>
To: Rusty Russell <rusty@rustcorp.com.au>
cc: nigel@nrg.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH for 2.5] preemptible kernel 
In-Reply-To: Your message of "Tue, 20 Mar 2001 19:43:50 +1100."
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Date: Tue, 20 Mar 2001 20:32:15 +1100
Message-ID: <851.985080735@ocs3.ocs-net>
Sender: linux-kernel-owner@vger.kernel.org

On Tue, 20 Mar 2001 19:43:50 +1100, 
Rusty Russell <rusty@rustcorp.com.au> wrote:
>The third is that preemtivity conflicts with the naive
>quiescent-period approach proposed for module unloading in 2.5, and
>useful for several other things (eg. hotplugging CPUs).  This method
>relies on knowing that when a schedule() has occurred on every CPU, we
>know noone is holding certain references.
>
>This, too, is soluble, but it means that synchronize_kernel() must
>guarantee that each task which was running or preempted in kernel
>space when it was called, has been non-preemtively scheduled before
>synchronize_kernel() can exit.  Icky.

The preemption patch only allows preemption from interrupt and only for
a single level of preemption.  That coexists quite happily with
synchronize_kernel() which runs in user context.  Just count user
context schedules (preempt_count == 0), not preemptive schedules.

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