[patch 00/12] arm: raw_spinlock annotations

Arnd Bergmann arnd at arndb.de
Tue Oct 19 16:20:32 EDT 2010

On Tuesday 19 October 2010 22:03:32 Thomas Gleixner wrote:
> On Tue, 19 Oct 2010, Russell King - ARM Linux wrote:
> > On Tue, Oct 19, 2010 at 05:26:45PM +0200, Arnd Bergmann wrote:
> > > On Tuesday 19 October 2010, Uwe Kleine-König wrote:
> > > > While cleaning up my repo I refound the patches and rebased them on top
> > > > of today's Linus' tree and only needed to fix up the l2x0_lock patch as
> > > > in the meantime a new usage hit mainline.
> > > 
> > > The patches all look harmless, but none of them has any information on
> > > why the particular locks need to be raw_spin_lock. Ideally a raw spinlock
> > > should be the absolute exception, and IMHO should have a comment in front
> > > of it why it is special.
> > 
> > Or at least explained in the patch description.
> > 
> > For instance, can someone explain why the lock for leds and gpio stuff
> > on Footbridge needs to be converted?  What is the original problem?
> > More importantly, what is the criteria for using a raw spinlock instead
> > of a normal spinlock?
> raw_spinlock is still a spinlock when PREEMPT_RT is enabled, mere
> spinlocks become magically "sleeping" spinlocks (aka. PI aware
> rtmutexes)

I think we all understood that part of the series description.

> Vs. the patches: IIRC, it was all fallout from running -rt, but that
> needs to be looked at case by case. Some of those are obvious as they
> are called deep down in atomic irq disabled code, but others might be
> just due to laziness reasons.

Not as obvious as you'd think. The explanation "called in atomic irq
disabled code" makes sense, but I hadn't thought of that before -- I had
expected all that code to use IRQ threads in case of PREEMPT_RT.

Requiring a comment there would probably eliminate the laziness issues,
adding a comment that you can't be bothered to do the right fix won't
help getting a patch merged, so ideally you'd end up with a better
solution getting merged.


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