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

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Date:	Thu, 02 Feb 2012 20:18:58 -0500 (EST)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	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, 2 Feb 2012, Stephen Boyd wrote:

> On 02/02/12 16:36, Russell King - ARM Linux wrote:
> > 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.
> 
> I'll try the macros. So far it isn't bad, just the __v7_setup to resolve.

If you simply disable/restore IRQs around the critical region then you 
don't have to worry about __v7_setup.  Plus this will allow for 
v7_flush_dcache_all to still be callable from atomic context.


Nicolas
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