linux-kernel - Re: [PATCH] ARM: cache-v7: Disable preemption when reading CCSIDR

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Date:	Fri, 3 Feb 2012 00:36:33 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Nicolas Pitre <nico@...xnic.net>,
	linux-arm-kernel@...ts.infradead.org,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH] ARM: cache-v7: Disable preemption when reading CCSIDR

On Thu, Feb 02, 2012 at 03:36:49PM -0800, Stephen Boyd wrote:
> On 02/02/12 13:38, Nicolas Pitre wrote:
> > On Thu, 2 Feb 2012, Russell King - ARM Linux wrote
> >> On Thu, Feb 02, 2012 at 11:24:46AM -0800, Stephen Boyd wrote:
> >>> Should we move get_thread_info into assembler.h? It seems odd
> >>> to include entry-header.S but I saw that vfp was doing the same.
> >> Probably yes, and probably also have preempt_disable and preempt_enable
> >> assembler macros.  That's going to get rather icky if we have to
> >> explicitly call the scheduler though (to solve (1)).
> > What about a pair of helpers written in C instead?
> >
> > v7_flush_dcache_all() could be renamed, and a wrapper function called 
> > v7_flush_dcache_all() would call the preemption disable helper, call the 
> > former v7_flush_dcache_all code, then call the preemption enable helper.
> >
> > Then __v7_setup() could still call the core cache flush code without 
> > issues.
> 
> I tried to put the preemption disable/enable right around the place
> where it was needed. With this approach we would disable preemption
> during the entire cache flush. I'm not sure if we want to make this
> function worse for performance, do we? It certainly sounds easier than
> writing all the preempt macros in assembly though.

Err, why do you think it's a big task?

preempt disable is a case of incrementing the thread preempt count, while
preempt enable is a case of decrementing it, testing for zero, if zero,
then checking whether TIF_NEED_RESCHED is set and calling a function.

If that's too much, then the simple method in assembly to quickly disable
preemption over a very few set of instructions is using mrs/msr and cpsid i.
That'll be far cheaper than fiddling about with preempt counters or
messing about with veneers in C code.
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