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Subject: Re: [PATCH] small fixes in brlock.h
From: Robert Love <rml@tech9.net>
To: Zwane Mwaikambo <zwane@linuxpower.ca>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>,
       Linus Torvalds <torvalds@transmeta.com>
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Date: 09 Mar 2003 19:00:01 -0500
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On Sun, 2003-03-09 at 18:44, Zwane Mwaikambo wrote:

> --- linux-2.5.64-unwashed/include/linux/brlock.h	5 Mar 2003 05:07:54 -0000	1.1.1.1
> +++ linux-2.5.64-unwashed/include/linux/brlock.h	9 Mar 2003 23:42:26 -0000
> @@ -85,8 +85,7 @@
>  	if (idx >= __BR_END)
>  		__br_lock_usage_bug();
>  
> -	preempt_disable();
> -	_raw_read_lock(&__brlock_array[smp_processor_id()][idx]);
> +	read_lock(&__brlock_array[smp_processor_id()][idx]);
>  }

This is wrong.

We have to disable preemption prior to reading smp_processor_id(). 
Otherwise we may lock/unlock the wrong processor's brlock.  The above as
you changed it is equivalent to:

	cpu = smp_processor_id();
	/* do not want to preempt here, but we can! */
	preempt_disable();
	_raw_read_lock(&__brlock_array[cpu][idx]);

And what we had, and what we need, is:

	preempt_disable();
	cpu = smp_processor_id();
	_raw_read_lock(&__brlock_array[cpu][idx]);

In other words, we need to ensure preemption is disabled prior to
calling smp_processor_id().

We also do something similar with the write patch in lib/brlock.c, but
for different reason: to disable preemption once and not NR_CPUS times.

The rest of the patch looks fine. :)

	Robert Love

-
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