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Date: Fri, 29 Oct 2004 23:51:22 -0500
From: Alan Cox <alc@cs.rice.edu>
To: Mike Silbersack <silby@silby.com>
Message-ID: <20041030045122.GK16728@cs.rice.edu>
References: <200410291910.i9TJAlNf089795@repoman.freebsd.org>
	<200410291549.17355.jhb@FreeBSD.org> <20041029174131.A6530@odysseus.silby.com>
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Subject: Re: cvs commit: src/sys/i386/i386 pmap.c
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On Fri, Oct 29, 2004 at 05:51:30PM -0500, Mike Silbersack wrote:
> 
[snip]
> 
> Instead, how about this - could per-CPU mutexes be implemented?  In that 
> mode, mutexes wouldn't use atomic ops, because they'd only ever be used by 
> a specific processor, thereby eliminating the need for atomic ops.  The 
> second modification would be that they set a per-CPU flag somewhere which 
> would tell the scheduler not to reawaken the sleeping kernel thread unless 
> it was on the same CPU it started on.

In a word, yes.  Start with the _get_sleep_lock macro from sys/mutex.h
and make two changes: (1) inline the implementation of sched_pin() just
before the _obtain_lock() call and (2) replace the current implementation
of _obtain_lock() with a new implementation that is atomic with respect
to interrupts but not other processors.

A similar approach could be applied to unlock, starting with the
_rel_sleep_lock() macro.

Alan