linux-kernel - Re: [PATCH] local_t protection (critical section)

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Message-ID: <20070912230037.GA1571@Krystal>
Date:	Wed, 12 Sep 2007 19:00:37 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <clameter@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, mingo@...hat.com,
	linux-ia64@...r.kernel.org
Subject: Re: [PATCH] local_t protection (critical section)

* Christoph Lameter (clameter@....com) wrote:
> On Wed, 5 Sep 2007, Mathieu Desnoyers wrote:
> 
> > Index: linux-2.6-lttng/include/asm-generic/local.h
> > ===================================================================
> > --- linux-2.6-lttng.orig/include/asm-generic/local.h	2007-09-04 15:32:02.000000000 -0400
> > +++ linux-2.6-lttng/include/asm-generic/local.h	2007-09-05 08:50:47.000000000 -0400
> > @@ -46,6 +46,15 @@ typedef struct
> >  #define local_add_unless(l, a, u) atomic_long_add_unless((&(l)->a), (a), (u))
> >  #define local_inc_not_zero(l) atomic_long_inc_not_zero(&(l)->a)
> >  
> > +#define local_enter_save(flags) local_irq_save(flags)
> > +#define local_exit_restore(flags) local_irq_restore(flags)
> > +#define local_enter() local_irq_disable()
> > +#define local_exit() local_irq_enable()
> > +#define local_nest_irq_save(flags) (flags)
> > +#define local_nest_irq_restore(flags) (flags)
> > +#define local_nest_irq_disable()
> > +#define local_nest_irq_enable()
> > +
> 
> This list is going to increase with RT support in SLUB? Argh.
> 

AFAIK, there is no difference between local irq save/restore in mainline
VS -RT. The same applies to preempt disable/enable.

The only thing we have to make sure is that the irq disable and
preempt disable code paths are short and O(1).

> 
> > Index: linux-2.6-lttng/include/asm-i386/local.h
> > ===================================================================
> > --- linux-2.6-lttng.orig/include/asm-i386/local.h	2007-09-04 15:28:52.000000000 -0400
> > +++ linux-2.6-lttng/include/asm-i386/local.h	2007-09-05 08:49:19.000000000 -0400
> > @@ -194,6 +194,23 @@ static __inline__ long local_sub_return(
> >  })
> >  #define local_inc_not_zero(l) local_add_unless((l), 1, 0)
> >  
> > +#define local_enter_save(flags) \
> > +	do { \
> > +		(flags); \
> > +		preempt_disable(); \
> > +	} while (0)
> 
> 
> > +#define local_exit_restore(flags) \
> > +	do { \
> > +		(flags); \
> > +		preempt_enable(); \
> > +	} while (0)
> 
> 
> This does not result in warnings because a variable is not used or used 
> uninitialized?

Because the variable is not used at all if I don't put the "(flags)"
(gcc warns about this).

I'm glad that some of the proposed changes may help. I'll let the
cmpxchg_local patches sleep for a while so I can concentrate my efforts
on text edit lock, immediate values and markers. I think what we'll
really need for the cmpxchg_local is two flavors: one that is as atomic
as possible (for things such as tracing), and the other one the fastest
possible (potentially using irq disable). A lot of per architecture
testing/fine tuning will be required though, and I don't have the
hardware to do this.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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