linux-kernel - Re: [PATCH] asm-generic: Use raw_local_irq_save/restore instead local_irq

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Message-Id: <20100526100805.171b3c48.akpm@linux-foundation.org>
Date:	Wed, 26 May 2010 10:08:05 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	rostedt@...dmis.org
Cc:	monstr@...str.eu, linux-kernel@...r.kernel.org,
	michal.simek@...alogix.com, arnd@...db.de,
	john.williams@...alogix.com, tglx@...utronix.de,
	peter.fritzsche@....de, anton@...ba.org, mingo@...e.hu
Subject: Re: [PATCH] asm-generic: Use raw_local_irq_save/restore instead
 local_irq_save/restore

On Wed, 26 May 2010 10:42:43 -0400 Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 2010-05-26 at 10:48 +0200, monstr@...str.eu wrote:
> > From: Michal Simek <monstr@...str.eu>
> > 
> > start/stop_critical_timing function for preemptirqsoff, preemptoff
> > and irqsoff tracers contains atomic_inc and atomic_dec operations.
> > 
> > Atomic operations used local_irq_save/restore macros to ensure
> > atomic access but they are traced by the same function which is causing
> > recursion problem.
> > 
> > The reason is when these tracers are turn ON then local_irq_save/restore
> > macros are changed in include/linux/irqflags.h to call trace_hardirqs_on/off
> > which call start/stop_critical_timing.
> > 
> > Microblaze was affected because use generic atomic implementation.
> > 
> > Signed-off-by: Michal Simek <monstr@...str.eu>
> 
> Acked-by: Steven Rostedt <rostedt@...dmis.org>
> 

Sighed-at-by: me.

> > diff --git a/include/asm-generic/atomic.h b/include/asm-generic/atomic.h
> > index 058129e..6c190fd 100644
> > --- a/include/asm-generic/atomic.h
> > +++ b/include/asm-generic/atomic.h
> > @@ -57,11 +57,11 @@ static inline int atomic_add_return(int i, atomic_t *v)
> >  	unsigned long flags;
> >  	int temp;
> >  
> > -	local_irq_save(flags);
> > +	raw_local_irq_save(flags);
> >  	temp = v->counter;
> >  	temp += i;
> >  	v->counter = temp;
> > -	local_irq_restore(flags);
> > +	raw_local_irq_restore(flags);
> >  
> >  	return temp;
> >  }

If a developer looks at atomic_add_return() and asks himself "why did
this use raw_local_irq_save()", the only way of answering that question
is to go groveling through the git logs, which is a right PITA if
you're trying to get some coding work done.

Guys, any time you add code which is non-obvious at the raw C level, it
*needs* a comment!
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