Re: [PATCH] local_t protection (critical section)

Mathieu Desnoyers
2007-09-12T23:00:37Z

* Christoph Lameter (clameter@sgi.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