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Date: Tue, 2 Oct 2007 20:00:22 -0700
From: "Kip Macy" <kip.macy@gmail.com>
To: "Alfred Perlstein" <alfred@freebsd.org>
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References: <20071003015231.GJ31826@elvis.mu.org>
	<Pine.GSO.4.64.0710022244250.626@sea.ntplx.net>
	<20071003025418.GN31826@elvis.mu.org>
Cc: Daniel Eischen <deischen@freebsd.org>, hackers@freebsd.org
Subject: Re: Critical Sections for userland.
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On 10/2/07, Alfred Perlstein <alfred@freebsd.org> wrote:
> * Daniel Eischen <deischen@freebsd.org> [071002 19:46] wrote:
> > On Tue, 2 Oct 2007, Alfred Perlstein wrote:
> >
> > >Hi guys, we need critical sections for userland here.
> > >
> > >This is basically to avoid a process being switched out while holding
> > >a user level spinlock.
> >
> > Setting the scheduling class to real-time and using SCHED_FIFO
> > and adjusting the thread priority around the lock doesn't work?
>
> Too heavy weight, we want to basically have this sort of code
> in userland:
>
> /* assume single threaded process for now */
> static int is_critical;
>
>
>
> atomic_mutex_lock();  /* implies ++is_critical */
>  ...do stuff...
> atomic_mutex_unlock(); /* implies --is_critical */
>
> We don't want two or more syscalls per lock operation. :)


I assume these processes are running as root? There is nothing to
prevent the process from never dropping the lock.

 -Kip